diff --git a/client_test.go b/client_test.go
new file mode 100644
index 0000000..d5bd971
--- /dev/null
+++ b/client_test.go
@@ -0,0 +1,406 @@
+package sendamatic
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestNewClient(t *testing.T) {
+	client := NewClient("test-user", "test-pass")
+
+	if client == nil {
+		t.Fatal("NewClient returned nil")
+	}
+
+	expectedAPIKey := "test-user-test-pass"
+	if client.apiKey != expectedAPIKey {
+		t.Errorf("apiKey = %q, want %q", client.apiKey, expectedAPIKey)
+	}
+
+	if client.baseURL != defaultBaseURL {
+		t.Errorf("baseURL = %q, want %q", client.baseURL, defaultBaseURL)
+	}
+
+	if client.httpClient == nil {
+		t.Fatal("httpClient is nil")
+	}
+
+	if client.httpClient.Timeout != defaultTimeout {
+		t.Errorf("httpClient.Timeout = %v, want %v", client.httpClient.Timeout, defaultTimeout)
+	}
+}
+
+func TestClient_Send_Success(t *testing.T) {
+	// Create a test server that returns a successful response
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		// Verify request method
+		if r.Method != http.MethodPost {
+			t.Errorf("Method = %s, want POST", r.Method)
+		}
+
+		// Verify request path
+		if r.URL.Path != "/send" {
+			t.Errorf("Path = %s, want /send", r.URL.Path)
+		}
+
+		// Verify headers
+		if ct := r.Header.Get("Content-Type"); ct != "application/json" {
+			t.Errorf("Content-Type = %s, want application/json", ct)
+		}
+
+		if apiKey := r.Header.Get("x-api-key"); apiKey != "user-pass" {
+			t.Errorf("x-api-key = %s, want user-pass", apiKey)
+		}
+
+		// Verify request body
+		body, _ := io.ReadAll(r.Body)
+		var msg Message
+		if err := json.Unmarshal(body, &msg); err != nil {
+			t.Errorf("Failed to unmarshal request body: %v", err)
+		}
+
+		// Send successful response
+		w.WriteHeader(http.StatusOK)
+		w.Header().Set("Content-Type", "application/json")
+		response := map[string][2]interface{}{
+			"recipient@example.com": {float64(200), "msg-12345"},
+		}
+		json.NewEncoder(w).Encode(response)
+	}))
+	defer server.Close()
+
+	client := NewClient("user", "pass", WithBaseURL(server.URL))
+
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		AddTo("recipient@example.com").
+		SetSubject("Test").
+		SetTextBody("Body")
+
+	resp, err := client.Send(context.Background(), msg)
+	if err != nil {
+		t.Fatalf("Send() error = %v, want nil", err)
+	}
+
+	if !resp.IsSuccess() {
+		t.Error("Expected successful response")
+	}
+
+	if resp.StatusCode != 200 {
+		t.Errorf("StatusCode = %d, want 200", resp.StatusCode)
+	}
+
+	msgID, ok := resp.GetMessageID("recipient@example.com")
+	if !ok {
+		t.Error("Expected to find message ID")
+	}
+	if msgID != "msg-12345" {
+		t.Errorf("MessageID = %q, want %q", msgID, "msg-12345")
+	}
+}
+
+func TestClient_Send_ValidationError(t *testing.T) {
+	client := NewClient("user", "pass")
+
+	// Create an invalid message (no recipients)
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		SetSubject("Test").
+		SetTextBody("Body")
+
+	_, err := client.Send(context.Background(), msg)
+	if err == nil {
+		t.Fatal("Expected validation error, got nil")
+	}
+
+	if !strings.Contains(err.Error(), "validation failed") {
+		t.Errorf("Error message = %q, want to contain 'validation failed'", err.Error())
+	}
+}
+
+func TestClient_Send_APIError(t *testing.T) {
+	tests := []struct {
+		name           string
+		statusCode     int
+		responseBody   string
+		wantErrMessage string
+	}{
+		{
+			name:           "400 bad request",
+			statusCode:     400,
+			responseBody:   `{"error": "Invalid request"}`,
+			wantErrMessage: "sendamatic api error (status 400): Invalid request",
+		},
+		{
+			name:           "401 unauthorized",
+			statusCode:     401,
+			responseBody:   `{"error": "Invalid API key"}`,
+			wantErrMessage: "sendamatic api error (status 401): Invalid API key",
+		},
+		{
+			name:           "422 validation error",
+			statusCode:     422,
+			responseBody:   `{"error": "Validation failed", "validation_errors": "sender is required", "json_path": "$.sender"}`,
+			wantErrMessage: "sendamatic api error (status 422): sender is required (path: $.sender)",
+		},
+		{
+			name:           "500 server error",
+			statusCode:     500,
+			responseBody:   `{"error": "Internal server error"}`,
+			wantErrMessage: "sendamatic api error (status 500): Internal server error",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				w.WriteHeader(tt.statusCode)
+				w.Write([]byte(tt.responseBody))
+			}))
+			defer server.Close()
+
+			client := NewClient("user", "pass", WithBaseURL(server.URL))
+
+			msg := NewMessage().
+				SetSender("sender@example.com").
+				AddTo("recipient@example.com").
+				SetSubject("Test").
+				SetTextBody("Body")
+
+			_, err := client.Send(context.Background(), msg)
+			if err == nil {
+				t.Fatal("Expected error, got nil")
+			}
+
+			var apiErr *APIError
+			if !errors.As(err, &apiErr) {
+				t.Fatalf("Error type = %T, want *APIError", err)
+			}
+
+			if apiErr.StatusCode != tt.statusCode {
+				t.Errorf("StatusCode = %d, want %d", apiErr.StatusCode, tt.statusCode)
+			}
+
+			if err.Error() != tt.wantErrMessage {
+				t.Errorf("Error message = %q, want %q", err.Error(), tt.wantErrMessage)
+			}
+		})
+	}
+}
+
+func TestClient_Send_ContextTimeout(t *testing.T) {
+	// Create a server that delays the response
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		time.Sleep(100 * time.Millisecond)
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"recipient@example.com": [200, "msg-12345"]}`))
+	}))
+	defer server.Close()
+
+	client := NewClient("user", "pass", WithBaseURL(server.URL))
+
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		AddTo("recipient@example.com").
+		SetSubject("Test").
+		SetTextBody("Body")
+
+	// Create a context that times out quickly
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Millisecond)
+	defer cancel()
+
+	_, err := client.Send(ctx, msg)
+	if err == nil {
+		t.Fatal("Expected timeout error, got nil")
+	}
+
+	if !errors.Is(err, context.DeadlineExceeded) && !strings.Contains(err.Error(), "context deadline exceeded") {
+		t.Errorf("Expected context deadline exceeded error, got: %v", err)
+	}
+}
+
+func TestClient_Send_ContextCancellation(t *testing.T) {
+	// Create a server that delays the response
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		time.Sleep(100 * time.Millisecond)
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"recipient@example.com": [200, "msg-12345"]}`))
+	}))
+	defer server.Close()
+
+	client := NewClient("user", "pass", WithBaseURL(server.URL))
+
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		AddTo("recipient@example.com").
+		SetSubject("Test").
+		SetTextBody("Body")
+
+	ctx, cancel := context.WithCancel(context.Background())
+
+	// Cancel the context immediately
+	cancel()
+
+	_, err := client.Send(ctx, msg)
+	if err == nil {
+		t.Fatal("Expected cancellation error, got nil")
+	}
+
+	if !errors.Is(err, context.Canceled) && !strings.Contains(err.Error(), "context canceled") {
+		t.Errorf("Expected context canceled error, got: %v", err)
+	}
+}
+
+func TestClient_Send_MultipleRecipients(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Header().Set("Content-Type", "application/json")
+		response := map[string][2]interface{}{
+			"recipient1@example.com": {float64(200), "msg-11111"},
+			"recipient2@example.com": {float64(200), "msg-22222"},
+			"recipient3@example.com": {float64(550), "msg-33333"}, // Failed delivery
+		}
+		json.NewEncoder(w).Encode(response)
+	}))
+	defer server.Close()
+
+	client := NewClient("user", "pass", WithBaseURL(server.URL))
+
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		AddTo("recipient1@example.com").
+		AddTo("recipient2@example.com").
+		AddTo("recipient3@example.com").
+		SetSubject("Test").
+		SetTextBody("Body")
+
+	resp, err := client.Send(context.Background(), msg)
+	if err != nil {
+		t.Fatalf("Send() error = %v, want nil", err)
+	}
+
+	// Check each recipient
+	for email, expected := range map[string]struct {
+		status int
+		msgID  string
+	}{
+		"recipient1@example.com": {200, "msg-11111"},
+		"recipient2@example.com": {200, "msg-22222"},
+		"recipient3@example.com": {550, "msg-33333"},
+	} {
+		status, ok := resp.GetStatus(email)
+		if !ok {
+			t.Errorf("Expected to find status for %s", email)
+			continue
+		}
+		if status != expected.status {
+			t.Errorf("Status for %s = %d, want %d", email, status, expected.status)
+		}
+
+		msgID, ok := resp.GetMessageID(email)
+		if !ok {
+			t.Errorf("Expected to find message ID for %s", email)
+			continue
+		}
+		if msgID != expected.msgID {
+			t.Errorf("MessageID for %s = %q, want %q", email, msgID, expected.msgID)
+		}
+	}
+}
+
+func TestClient_Send_InvalidJSON(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`not valid json`))
+	}))
+	defer server.Close()
+
+	client := NewClient("user", "pass", WithBaseURL(server.URL))
+
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		AddTo("recipient@example.com").
+		SetSubject("Test").
+		SetTextBody("Body")
+
+	_, err := client.Send(context.Background(), msg)
+	if err == nil {
+		t.Fatal("Expected error for invalid JSON, got nil")
+	}
+
+	if !strings.Contains(err.Error(), "unmarshal") {
+		t.Errorf("Error should mention unmarshal, got: %v", err)
+	}
+}
+
+func TestClient_Send_EmptyResponse(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{}`))
+	}))
+	defer server.Close()
+
+	client := NewClient("user", "pass", WithBaseURL(server.URL))
+
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		AddTo("recipient@example.com").
+		SetSubject("Test").
+		SetTextBody("Body")
+
+	resp, err := client.Send(context.Background(), msg)
+	if err != nil {
+		t.Fatalf("Send() error = %v, want nil", err)
+	}
+
+	if resp.StatusCode != 200 {
+		t.Errorf("StatusCode = %d, want 200", resp.StatusCode)
+	}
+
+	if len(resp.Recipients) != 0 {
+		t.Errorf("Recipients length = %d, want 0", len(resp.Recipients))
+	}
+}
+
+func TestClient_Send_WithAttachments(t *testing.T) {
+	var receivedMsg Message
+
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, _ := io.ReadAll(r.Body)
+		json.Unmarshal(body, &receivedMsg)
+
+		w.WriteHeader(http.StatusOK)
+		w.Write([]byte(`{"recipient@example.com": [200, "msg-12345"]}`))
+	}))
+	defer server.Close()
+
+	client := NewClient("user", "pass", WithBaseURL(server.URL))
+
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		AddTo("recipient@example.com").
+		SetSubject("Test").
+		SetTextBody("Body").
+		AttachFile("test.txt", "text/plain", []byte("test content"))
+
+	_, err := client.Send(context.Background(), msg)
+	if err != nil {
+		t.Fatalf("Send() error = %v, want nil", err)
+	}
+
+	// Verify attachment was sent
+	if len(receivedMsg.Attachments) != 1 {
+		t.Fatalf("Attachments length = %d, want 1", len(receivedMsg.Attachments))
+	}
+
+	if receivedMsg.Attachments[0].Filename != "test.txt" {
+		t.Errorf("Filename = %q, want %q", receivedMsg.Attachments[0].Filename, "test.txt")
+	}
+}
diff --git a/errors_test.go b/errors_test.go
new file mode 100644
index 0000000..6e9b0ac
--- /dev/null
+++ b/errors_test.go
@@ -0,0 +1,229 @@
+package sendamatic
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+func TestAPIError_Error(t *testing.T) {
+	tests := []struct {
+		name     string
+		apiErr   *APIError
+		wantText string
+	}{
+		{
+			name: "simple error",
+			apiErr: &APIError{
+				StatusCode: 400,
+				Message:    "Invalid request",
+			},
+			wantText: "sendamatic api error (status 400): Invalid request",
+		},
+		{
+			name: "error with validation details",
+			apiErr: &APIError{
+				StatusCode:       422,
+				Message:          "Validation failed",
+				ValidationErrors: "sender is required",
+				JSONPath:         "$.sender",
+			},
+			wantText: "sendamatic api error (status 422): sender is required (path: $.sender)",
+		},
+		{
+			name: "error with SMTP code",
+			apiErr: &APIError{
+				StatusCode: 500,
+				Message:    "SMTP error",
+				SMTPCode:   550,
+				Sender:     "test@example.com",
+			},
+			wantText: "sendamatic api error (status 500): SMTP error",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got := tt.apiErr.Error()
+			if got != tt.wantText {
+				t.Errorf("Error() = %q, want %q", got, tt.wantText)
+			}
+		})
+	}
+}
+
+func TestParseErrorResponse_ValidJSON(t *testing.T) {
+	tests := []struct {
+		name       string
+		statusCode int
+		body       string
+		want       *APIError
+	}{
+		{
+			name:       "simple error",
+			statusCode: 400,
+			body:       `{"error": "Invalid API key"}`,
+			want: &APIError{
+				StatusCode: 400,
+				Message:    "Invalid API key",
+			},
+		},
+		{
+			name:       "validation error",
+			statusCode: 422,
+			body:       `{"error": "Validation failed", "validation_errors": "sender is required", "json_path": "$.sender"}`,
+			want: &APIError{
+				StatusCode:       422,
+				Message:          "Validation failed",
+				ValidationErrors: "sender is required",
+				JSONPath:         "$.sender",
+			},
+		},
+		{
+			name:       "smtp error",
+			statusCode: 500,
+			body:       `{"error": "SMTP error", "smtp_code": 550, "sender": "test@example.com"}`,
+			want: &APIError{
+				StatusCode: 500,
+				Message:    "SMTP error",
+				SMTPCode:   550,
+				Sender:     "test@example.com",
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := parseErrorResponse(tt.statusCode, []byte(tt.body))
+
+			apiErr, ok := err.(*APIError)
+			if !ok {
+				t.Fatalf("parseErrorResponse returned %T, want *APIError", err)
+			}
+
+			if apiErr.StatusCode != tt.want.StatusCode {
+				t.Errorf("StatusCode = %d, want %d", apiErr.StatusCode, tt.want.StatusCode)
+			}
+
+			if apiErr.Message != tt.want.Message {
+				t.Errorf("Message = %q, want %q", apiErr.Message, tt.want.Message)
+			}
+
+			if apiErr.ValidationErrors != tt.want.ValidationErrors {
+				t.Errorf("ValidationErrors = %q, want %q", apiErr.ValidationErrors, tt.want.ValidationErrors)
+			}
+
+			if apiErr.JSONPath != tt.want.JSONPath {
+				t.Errorf("JSONPath = %q, want %q", apiErr.JSONPath, tt.want.JSONPath)
+			}
+
+			if apiErr.SMTPCode != tt.want.SMTPCode {
+				t.Errorf("SMTPCode = %d, want %d", apiErr.SMTPCode, tt.want.SMTPCode)
+			}
+
+			if apiErr.Sender != tt.want.Sender {
+				t.Errorf("Sender = %q, want %q", apiErr.Sender, tt.want.Sender)
+			}
+		})
+	}
+}
+
+func TestParseErrorResponse_InvalidJSON(t *testing.T) {
+	statusCode := 500
+	body := []byte("Internal Server Error - not JSON")
+
+	err := parseErrorResponse(statusCode, body)
+
+	apiErr, ok := err.(*APIError)
+	if !ok {
+		t.Fatalf("parseErrorResponse returned %T, want *APIError", err)
+	}
+
+	if apiErr.StatusCode != statusCode {
+		t.Errorf("StatusCode = %d, want %d", apiErr.StatusCode, statusCode)
+	}
+
+	// When JSON parsing fails, the raw body should be used as the message
+	if apiErr.Message != string(body) {
+		t.Errorf("Message = %q, want %q", apiErr.Message, string(body))
+	}
+}
+
+func TestParseErrorResponse_EmptyBody(t *testing.T) {
+	statusCode := 404
+	body := []byte("")
+
+	err := parseErrorResponse(statusCode, body)
+
+	apiErr, ok := err.(*APIError)
+	if !ok {
+		t.Fatalf("parseErrorResponse returned %T, want *APIError", err)
+	}
+
+	if apiErr.StatusCode != statusCode {
+		t.Errorf("StatusCode = %d, want %d", apiErr.StatusCode, statusCode)
+	}
+}
+
+func TestParseErrorResponse_MalformedJSON(t *testing.T) {
+	statusCode := 400
+	body := []byte(`{"error": "Missing closing brace"`)
+
+	err := parseErrorResponse(statusCode, body)
+
+	apiErr, ok := err.(*APIError)
+	if !ok {
+		t.Fatalf("parseErrorResponse returned %T, want *APIError", err)
+	}
+
+	if apiErr.StatusCode != statusCode {
+		t.Errorf("StatusCode = %d, want %d", apiErr.StatusCode, statusCode)
+	}
+
+	// Should fall back to raw body as message
+	if apiErr.Message != string(body) {
+		t.Errorf("Message = %q, want %q", apiErr.Message, string(body))
+	}
+}
+
+func TestAPIError_JSONRoundtrip(t *testing.T) {
+	original := &APIError{
+		StatusCode:       422,
+		Message:          "Validation error",
+		ValidationErrors: "sender format invalid",
+		JSONPath:         "$.sender",
+		Sender:           "invalid@",
+		SMTPCode:         0,
+	}
+
+	// Marshal to JSON
+	data, err := json.Marshal(original)
+	if err != nil {
+		t.Fatalf("Marshal failed: %v", err)
+	}
+
+	// Unmarshal back
+	var decoded APIError
+	err = json.Unmarshal(data, &decoded)
+	if err != nil {
+		t.Fatalf("Unmarshal failed: %v", err)
+	}
+
+	// StatusCode should not be in JSON (json:"-" tag)
+	if strings.Contains(string(data), "StatusCode") {
+		t.Error("StatusCode should not be marshaled to JSON")
+	}
+
+	// Compare fields (except StatusCode which has json:"-")
+	if decoded.Message != original.Message {
+		t.Errorf("Message = %q, want %q", decoded.Message, original.Message)
+	}
+
+	if decoded.ValidationErrors != original.ValidationErrors {
+		t.Errorf("ValidationErrors = %q, want %q", decoded.ValidationErrors, original.ValidationErrors)
+	}
+
+	if decoded.JSONPath != original.JSONPath {
+		t.Errorf("JSONPath = %q, want %q", decoded.JSONPath, original.JSONPath)
+	}
+}
diff --git a/message_test.go b/message_test.go
new file mode 100644
index 0000000..5a432b8
--- /dev/null
+++ b/message_test.go
@@ -0,0 +1,285 @@
+package sendamatic
+
+import (
+	"encoding/base64"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestNewMessage(t *testing.T) {
+	msg := NewMessage()
+
+	if msg == nil {
+		t.Fatal("NewMessage returned nil")
+	}
+
+	if msg.To == nil || msg.CC == nil || msg.BCC == nil {
+		t.Error("Slices not initialized")
+	}
+
+	if msg.Headers == nil || msg.Attachments == nil {
+		t.Error("Headers or Attachments not initialized")
+	}
+}
+
+func TestMessageBuilderMethods(t *testing.T) {
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		AddTo("to@example.com").
+		AddCC("cc@example.com").
+		AddBCC("bcc@example.com").
+		SetSubject("Test Subject").
+		SetTextBody("Test Body").
+		SetHTMLBody("<p>Test Body</p>").
+		AddHeader("X-Custom", "value")
+
+	if msg.Sender != "sender@example.com" {
+		t.Errorf("Sender = %q, want %q", msg.Sender, "sender@example.com")
+	}
+
+	if len(msg.To) != 1 || msg.To[0] != "to@example.com" {
+		t.Errorf("To = %v, want [to@example.com]", msg.To)
+	}
+
+	if len(msg.CC) != 1 || msg.CC[0] != "cc@example.com" {
+		t.Errorf("CC = %v, want [cc@example.com]", msg.CC)
+	}
+
+	if len(msg.BCC) != 1 || msg.BCC[0] != "bcc@example.com" {
+		t.Errorf("BCC = %v, want [bcc@example.com]", msg.BCC)
+	}
+
+	if msg.Subject != "Test Subject" {
+		t.Errorf("Subject = %q, want %q", msg.Subject, "Test Subject")
+	}
+
+	if msg.TextBody != "Test Body" {
+		t.Errorf("TextBody = %q, want %q", msg.TextBody, "Test Body")
+	}
+
+	if msg.HTMLBody != "<p>Test Body</p>" {
+		t.Errorf("HTMLBody = %q, want %q", msg.HTMLBody, "<p>Test Body</p>")
+	}
+
+	if len(msg.Headers) != 1 {
+		t.Fatalf("Headers length = %d, want 1", len(msg.Headers))
+	}
+
+	if msg.Headers[0].Header != "X-Custom" || msg.Headers[0].Value != "value" {
+		t.Errorf("Header = %+v, want {X-Custom value}", msg.Headers[0])
+	}
+}
+
+func TestAddMultipleRecipients(t *testing.T) {
+	msg := NewMessage().
+		AddTo("to1@example.com").
+		AddTo("to2@example.com").
+		AddTo("to3@example.com")
+
+	if len(msg.To) != 3 {
+		t.Errorf("To length = %d, want 3", len(msg.To))
+	}
+
+	expected := []string{"to1@example.com", "to2@example.com", "to3@example.com"}
+	for i, email := range expected {
+		if msg.To[i] != email {
+			t.Errorf("To[%d] = %q, want %q", i, msg.To[i], email)
+		}
+	}
+}
+
+func TestAttachFile(t *testing.T) {
+	msg := NewMessage()
+	data := []byte("test file content")
+
+	msg.AttachFile("test.txt", "text/plain", data)
+
+	if len(msg.Attachments) != 1 {
+		t.Fatalf("Attachments length = %d, want 1", len(msg.Attachments))
+	}
+
+	att := msg.Attachments[0]
+	if att.Filename != "test.txt" {
+		t.Errorf("Filename = %q, want %q", att.Filename, "test.txt")
+	}
+
+	if att.MimeType != "text/plain" {
+		t.Errorf("MimeType = %q, want %q", att.MimeType, "text/plain")
+	}
+
+	// Verify base64 encoding
+	decoded, err := base64.StdEncoding.DecodeString(att.Data)
+	if err != nil {
+		t.Fatalf("Failed to decode base64: %v", err)
+	}
+
+	if string(decoded) != string(data) {
+		t.Errorf("Decoded data = %q, want %q", decoded, data)
+	}
+}
+
+func TestAttachFileFromPath(t *testing.T) {
+	msg := NewMessage()
+
+	testFile := filepath.Join("testdata", "test.txt")
+	err := msg.AttachFileFromPath(testFile, "text/plain")
+	if err != nil {
+		t.Fatalf("AttachFileFromPath failed: %v", err)
+	}
+
+	if len(msg.Attachments) != 1 {
+		t.Fatalf("Attachments length = %d, want 1", len(msg.Attachments))
+	}
+
+	att := msg.Attachments[0]
+	if att.Filename != "test.txt" {
+		t.Errorf("Filename = %q, want %q", att.Filename, "test.txt")
+	}
+
+	// Verify content
+	decoded, err := base64.StdEncoding.DecodeString(att.Data)
+	if err != nil {
+		t.Fatalf("Failed to decode base64: %v", err)
+	}
+
+	expected, _ := os.ReadFile(testFile)
+	if string(decoded) != string(expected) {
+		t.Errorf("File content mismatch")
+	}
+}
+
+func TestAttachFileFromPath_NonExistent(t *testing.T) {
+	msg := NewMessage()
+
+	err := msg.AttachFileFromPath("nonexistent.txt", "text/plain")
+	if err == nil {
+		t.Error("Expected error for non-existent file, got nil")
+	}
+}
+
+func TestAttachMultipleFiles(t *testing.T) {
+	msg := NewMessage().
+		AttachFile("file1.txt", "text/plain", []byte("content1")).
+		AttachFile("file2.pdf", "application/pdf", []byte("content2"))
+
+	if len(msg.Attachments) != 2 {
+		t.Errorf("Attachments length = %d, want 2", len(msg.Attachments))
+	}
+}
+
+func TestValidate_Success(t *testing.T) {
+	tests := []struct {
+		name string
+		msg  *Message
+	}{
+		{
+			name: "valid with text body",
+			msg: NewMessage().
+				SetSender("sender@example.com").
+				AddTo("to@example.com").
+				SetSubject("Subject").
+				SetTextBody("Body"),
+		},
+		{
+			name: "valid with html body",
+			msg: NewMessage().
+				SetSender("sender@example.com").
+				AddTo("to@example.com").
+				SetSubject("Subject").
+				SetHTMLBody("<p>Body</p>"),
+		},
+		{
+			name: "valid with both bodies",
+			msg: NewMessage().
+				SetSender("sender@example.com").
+				AddTo("to@example.com").
+				SetSubject("Subject").
+				SetTextBody("Body").
+				SetHTMLBody("<p>Body</p>"),
+		},
+		{
+			name: "valid with multiple recipients",
+			msg: NewMessage().
+				SetSender("sender@example.com").
+				AddTo("to1@example.com").
+				AddTo("to2@example.com").
+				AddCC("cc@example.com").
+				AddBCC("bcc@example.com").
+				SetSubject("Subject").
+				SetTextBody("Body"),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.msg.Validate()
+			if err != nil {
+				t.Errorf("Validate() error = %v, want nil", err)
+			}
+		})
+	}
+}
+
+func TestValidate_Errors(t *testing.T) {
+	tests := []struct {
+		name        string
+		msg         *Message
+		wantErrText string
+	}{
+		{
+			name:        "no recipients",
+			msg:         NewMessage().SetSender("sender@example.com").SetSubject("Subject").SetTextBody("Body"),
+			wantErrText: "at least one recipient required",
+		},
+		{
+			name:        "no sender",
+			msg:         NewMessage().AddTo("to@example.com").SetSubject("Subject").SetTextBody("Body"),
+			wantErrText: "sender is required",
+		},
+		{
+			name:        "no subject",
+			msg:         NewMessage().SetSender("sender@example.com").AddTo("to@example.com").SetTextBody("Body"),
+			wantErrText: "subject is required",
+		},
+		{
+			name:        "no body",
+			msg:         NewMessage().SetSender("sender@example.com").AddTo("to@example.com").SetSubject("Subject"),
+			wantErrText: "either text_body or html_body is required",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.msg.Validate()
+			if err == nil {
+				t.Fatal("Validate() error = nil, want error")
+			}
+			if err.Error() != tt.wantErrText {
+				t.Errorf("Validate() error = %q, want %q", err.Error(), tt.wantErrText)
+			}
+		})
+	}
+}
+
+func TestValidate_TooManyRecipients(t *testing.T) {
+	msg := NewMessage().
+		SetSender("sender@example.com").
+		SetSubject("Subject").
+		SetTextBody("Body")
+
+	// Add 256 recipients (more than the limit of 255)
+	for i := 0; i < 256; i++ {
+		msg.AddTo("recipient@example.com")
+	}
+
+	err := msg.Validate()
+	if err == nil {
+		t.Fatal("Validate() error = nil, want error for too many recipients")
+	}
+
+	expected := "maximum 255 recipients allowed"
+	if err.Error() != expected {
+		t.Errorf("Validate() error = %q, want %q", err.Error(), expected)
+	}
+}
diff --git a/options_test.go b/options_test.go
new file mode 100644
index 0000000..1f235c3
--- /dev/null
+++ b/options_test.go
@@ -0,0 +1,154 @@
+package sendamatic
+
+import (
+	"net/http"
+	"testing"
+	"time"
+)
+
+func TestWithBaseURL(t *testing.T) {
+	customURL := "https://custom.api.url"
+	client := NewClient("user", "pass", WithBaseURL(customURL))
+
+	if client.baseURL != customURL {
+		t.Errorf("baseURL = %q, want %q", client.baseURL, customURL)
+	}
+}
+
+func TestWithTimeout(t *testing.T) {
+	customTimeout := 60 * time.Second
+	client := NewClient("user", "pass", WithTimeout(customTimeout))
+
+	if client.httpClient.Timeout != customTimeout {
+		t.Errorf("httpClient.Timeout = %v, want %v", client.httpClient.Timeout, customTimeout)
+	}
+}
+
+func TestWithHTTPClient(t *testing.T) {
+	customClient := &http.Client{
+		Timeout: 90 * time.Second,
+	}
+
+	client := NewClient("user", "pass", WithHTTPClient(customClient))
+
+	if client.httpClient != customClient {
+		t.Error("httpClient not set to custom client")
+	}
+
+	if client.httpClient.Timeout != 90*time.Second {
+		t.Errorf("httpClient.Timeout = %v, want 90s", client.httpClient.Timeout)
+	}
+}
+
+func TestMultipleOptions(t *testing.T) {
+	customURL := "https://test.api.url"
+	customTimeout := 45 * time.Second
+
+	client := NewClient("user", "pass",
+		WithBaseURL(customURL),
+		WithTimeout(customTimeout),
+	)
+
+	if client.baseURL != customURL {
+		t.Errorf("baseURL = %q, want %q", client.baseURL, customURL)
+	}
+
+	if client.httpClient.Timeout != customTimeout {
+		t.Errorf("httpClient.Timeout = %v, want %v", client.httpClient.Timeout, customTimeout)
+	}
+}
+
+func TestDefaultValues(t *testing.T) {
+	client := NewClient("user", "pass")
+
+	if client.baseURL != defaultBaseURL {
+		t.Errorf("baseURL = %q, want %q", client.baseURL, defaultBaseURL)
+	}
+
+	if client.httpClient.Timeout != defaultTimeout {
+		t.Errorf("httpClient.Timeout = %v, want %v", client.httpClient.Timeout, defaultTimeout)
+	}
+
+	expectedAPIKey := "user-pass"
+	if client.apiKey != expectedAPIKey {
+		t.Errorf("apiKey = %q, want %q", client.apiKey, expectedAPIKey)
+	}
+}
+
+func TestWithHTTPClient_PreservesCustomTransport(t *testing.T) {
+	customTransport := &http.Transport{
+		MaxIdleConns: 100,
+	}
+
+	customClient := &http.Client{
+		Timeout:   45 * time.Second,
+		Transport: customTransport,
+	}
+
+	client := NewClient("user", "pass", WithHTTPClient(customClient))
+
+	if client.httpClient.Transport != customTransport {
+		t.Error("Custom transport was not preserved")
+	}
+}
+
+func TestOptionsOrder(t *testing.T) {
+	// Test that options are applied in order
+	// First set timeout to 30s, then provide a custom client with 60s
+	customClient := &http.Client{
+		Timeout: 60 * time.Second,
+	}
+
+	client := NewClient("user", "pass",
+		WithTimeout(30*time.Second),
+		WithHTTPClient(customClient),
+	)
+
+	// The custom client should override the previous timeout setting
+	if client.httpClient.Timeout != 60*time.Second {
+		t.Errorf("httpClient.Timeout = %v, want 60s (custom client should override)", client.httpClient.Timeout)
+	}
+}
+
+func TestWithTimeout_OverridesDefault(t *testing.T) {
+	tests := []struct {
+		name    string
+		timeout time.Duration
+	}{
+		{"short timeout", 5 * time.Second},
+		{"long timeout", 120 * time.Second},
+		{"zero timeout", 0},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			client := NewClient("user", "pass", WithTimeout(tt.timeout))
+
+			if client.httpClient.Timeout != tt.timeout {
+				t.Errorf("httpClient.Timeout = %v, want %v", client.httpClient.Timeout, tt.timeout)
+			}
+		})
+	}
+}
+
+func TestWithBaseURL_VariousURLs(t *testing.T) {
+	tests := []struct {
+		name string
+		url  string
+	}{
+		{"production", "https://send.api.sendamatic.net"},
+		{"staging", "https://staging.api.sendamatic.net"},
+		{"local", "http://localhost:8080"},
+		{"custom port", "https://api.example.com:8443"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			client := NewClient("user", "pass", WithBaseURL(tt.url))
+
+			if client.baseURL != tt.url {
+				t.Errorf("baseURL = %q, want %q", client.baseURL, tt.url)
+			}
+		})
+	}
+}
diff --git a/response_test.go b/response_test.go
new file mode 100644
index 0000000..a9efac3
--- /dev/null
+++ b/response_test.go
@@ -0,0 +1,276 @@
+package sendamatic
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestSendResponse_IsSuccess(t *testing.T) {
+	tests := []struct {
+		name       string
+		statusCode int
+		want       bool
+	}{
+		{"success 200", 200, true},
+		{"bad request 400", 400, false},
+		{"unauthorized 401", 401, false},
+		{"server error 500", 500, false},
+		{"created 201", 201, false}, // Only 200 is considered success
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			resp := &SendResponse{StatusCode: tt.statusCode}
+			got := resp.IsSuccess()
+			if got != tt.want {
+				t.Errorf("IsSuccess() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestSendResponse_GetMessageID(t *testing.T) {
+	tests := []struct {
+		name       string
+		recipients map[string][2]interface{}
+		email      string
+		wantID     string
+		wantOK     bool
+	}{
+		{
+			name: "existing recipient",
+			recipients: map[string][2]interface{}{
+				"test@example.com": {float64(200), "msg-12345"},
+			},
+			email:  "test@example.com",
+			wantID: "msg-12345",
+			wantOK: true,
+		},
+		{
+			name: "non-existent recipient",
+			recipients: map[string][2]interface{}{
+				"test@example.com": {float64(200), "msg-12345"},
+			},
+			email:  "other@example.com",
+			wantID: "",
+			wantOK: false,
+		},
+		{
+			name: "multiple recipients",
+			recipients: map[string][2]interface{}{
+				"test1@example.com": {float64(200), "msg-11111"},
+				"test2@example.com": {float64(200), "msg-22222"},
+				"test3@example.com": {float64(400), "msg-33333"},
+			},
+			email:  "test2@example.com",
+			wantID: "msg-22222",
+			wantOK: true,
+		},
+		{
+			name:       "empty recipients",
+			recipients: map[string][2]interface{}{},
+			email:      "test@example.com",
+			wantID:     "",
+			wantOK:     false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			resp := &SendResponse{
+				StatusCode: 200,
+				Recipients: tt.recipients,
+			}
+
+			gotID, gotOK := resp.GetMessageID(tt.email)
+			if gotID != tt.wantID {
+				t.Errorf("GetMessageID() id = %q, want %q", gotID, tt.wantID)
+			}
+			if gotOK != tt.wantOK {
+				t.Errorf("GetMessageID() ok = %v, want %v", gotOK, tt.wantOK)
+			}
+		})
+	}
+}
+
+func TestSendResponse_GetStatus(t *testing.T) {
+	tests := []struct {
+		name       string
+		recipients map[string][2]interface{}
+		email      string
+		wantStatus int
+		wantOK     bool
+	}{
+		{
+			name: "existing recipient with success",
+			recipients: map[string][2]interface{}{
+				"test@example.com": {float64(200), "msg-12345"},
+			},
+			email:      "test@example.com",
+			wantStatus: 200,
+			wantOK:     true,
+		},
+		{
+			name: "existing recipient with error",
+			recipients: map[string][2]interface{}{
+				"test@example.com": {float64(400), "msg-12345"},
+			},
+			email:      "test@example.com",
+			wantStatus: 400,
+			wantOK:     true,
+		},
+		{
+			name: "non-existent recipient",
+			recipients: map[string][2]interface{}{
+				"test@example.com": {float64(200), "msg-12345"},
+			},
+			email:      "other@example.com",
+			wantStatus: 0,
+			wantOK:     false,
+		},
+		{
+			name: "multiple recipients",
+			recipients: map[string][2]interface{}{
+				"test1@example.com": {float64(200), "msg-11111"},
+				"test2@example.com": {float64(550), "msg-22222"},
+				"test3@example.com": {float64(200), "msg-33333"},
+			},
+			email:      "test2@example.com",
+			wantStatus: 550,
+			wantOK:     true,
+		},
+		{
+			name:       "empty recipients",
+			recipients: map[string][2]interface{}{},
+			email:      "test@example.com",
+			wantStatus: 0,
+			wantOK:     false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			resp := &SendResponse{
+				StatusCode: 200,
+				Recipients: tt.recipients,
+			}
+
+			gotStatus, gotOK := resp.GetStatus(tt.email)
+			if gotStatus != tt.wantStatus {
+				t.Errorf("GetStatus() status = %d, want %d", gotStatus, tt.wantStatus)
+			}
+			if gotOK != tt.wantOK {
+				t.Errorf("GetStatus() ok = %v, want %v", gotOK, tt.wantOK)
+			}
+		})
+	}
+}
+
+func TestSendResponse_JSONUnmarshal(t *testing.T) {
+	// Test that we can properly unmarshal the API response format
+	jsonResp := `{
+		"test1@example.com": [200, "msg-11111"],
+		"test2@example.com": [400, "msg-22222"]
+	}`
+
+	var recipients map[string][2]interface{}
+	err := json.Unmarshal([]byte(jsonResp), &recipients)
+	if err != nil {
+		t.Fatalf("Unmarshal failed: %v", err)
+	}
+
+	resp := &SendResponse{
+		StatusCode: 200,
+		Recipients: recipients,
+	}
+
+	// Test first recipient
+	status1, ok1 := resp.GetStatus("test1@example.com")
+	if !ok1 {
+		t.Error("Expected to find test1@example.com")
+	}
+	if status1 != 200 {
+		t.Errorf("Status for test1 = %d, want 200", status1)
+	}
+
+	msgID1, ok1 := resp.GetMessageID("test1@example.com")
+	if !ok1 {
+		t.Error("Expected to find message ID for test1@example.com")
+	}
+	if msgID1 != "msg-11111" {
+		t.Errorf("MessageID for test1 = %q, want %q", msgID1, "msg-11111")
+	}
+
+	// Test second recipient
+	status2, ok2 := resp.GetStatus("test2@example.com")
+	if !ok2 {
+		t.Error("Expected to find test2@example.com")
+	}
+	if status2 != 400 {
+		t.Errorf("Status for test2 = %d, want 400", status2)
+	}
+
+	msgID2, ok2 := resp.GetMessageID("test2@example.com")
+	if !ok2 {
+		t.Error("Expected to find message ID for test2@example.com")
+	}
+	if msgID2 != "msg-22222" {
+		t.Errorf("MessageID for test2 = %q, want %q", msgID2, "msg-22222")
+	}
+}
+
+func TestSendResponse_GetStatus_Float64Conversion(t *testing.T) {
+	// Explicitly test the float64 to int conversion
+	// This mimics how JSON unmarshaling works with numbers
+	resp := &SendResponse{
+		StatusCode: 200,
+		Recipients: map[string][2]interface{}{
+			"test@example.com": {float64(200.0), "msg-12345"},
+		},
+	}
+
+	status, ok := resp.GetStatus("test@example.com")
+	if !ok {
+		t.Fatal("Expected to find recipient")
+	}
+
+	if status != 200 {
+		t.Errorf("GetStatus() = %d, want 200", status)
+	}
+}
+
+func TestSendResponse_GetMessageID_InvalidType(t *testing.T) {
+	// Test behavior when message ID is not a string
+	resp := &SendResponse{
+		StatusCode: 200,
+		Recipients: map[string][2]interface{}{
+			"test@example.com": {float64(200), 12345}, // number instead of string
+		},
+	}
+
+	msgID, ok := resp.GetMessageID("test@example.com")
+	if ok {
+		t.Error("Expected ok = false when message ID is not a string")
+	}
+	if msgID != "" {
+		t.Errorf("Expected empty string, got %q", msgID)
+	}
+}
+
+func TestSendResponse_GetStatus_InvalidType(t *testing.T) {
+	// Test behavior when status is not a number
+	resp := &SendResponse{
+		StatusCode: 200,
+		Recipients: map[string][2]interface{}{
+			"test@example.com": {"OK", "msg-12345"}, // string instead of number
+		},
+	}
+
+	status, ok := resp.GetStatus("test@example.com")
+	if ok {
+		t.Error("Expected ok = false when status is not a number")
+	}
+	if status != 0 {
+		t.Errorf("Expected 0, got %d", status)
+	}
+}
diff --git a/testdata/test.pdf b/testdata/test.pdf
new file mode 100644
index 0000000..49efd7f
--- /dev/null
+++ b/testdata/test.pdf
@@ -0,0 +1,58 @@
+%PDF-1.4
+1 0 obj
+<<
+/Type /Catalog
+/Pages 2 0 R
+>>
+endobj
+2 0 obj
+<<
+/Type /Pages
+/Kids [3 0 R]
+/Count 1
+>>
+endobj
+3 0 obj
+<<
+/Type /Page
+/Parent 2 0 R
+/Resources <<
+/Font <<
+/F1 <<
+/Type /Font
+/Subtype /Type1
+/BaseFont /Helvetica
+>>
+>>
+>>
+/MediaBox [0 0 612 792]
+/Contents 4 0 R
+>>
+endobj
+4 0 obj
+<<
+/Length 44
+>>
+stream
+BT
+/F1 12 Tf
+100 700 Td
+(Test PDF) Tj
+ET
+endstream
+endobj
+xref
+0 5
+0000000000 65535 f
+0000000009 00000 n
+0000000058 00000 n
+0000000115 00000 n
+0000000317 00000 n
+trailer
+<<
+/Size 5
+/Root 1 0 R
+>>
+startxref
+408
+%%EOF
\ No newline at end of file
diff --git a/testdata/test.png b/testdata/test.png
new file mode 100644
index 0000000..91a99b9
Binary files /dev/null and b/testdata/test.png differ
diff --git a/testdata/test.txt b/testdata/test.txt
new file mode 100644
index 0000000..100ff51
--- /dev/null
+++ b/testdata/test.txt
@@ -0,0 +1 @@
+This is a test text file for attachment testing.