feat: add Email Plugin with SMTP functionality

- Implemented the Email Plugin for self-hosted Memos instances, providing SMTP email sending capabilities.
- Created configuration structure for SMTP settings with validation.
- Developed message structure for email content with validation and formatting.
- Added synchronous and asynchronous email sending methods.
- Implemented error handling and logging for email sending processes.
- Included tests for client, configuration, and message functionalities to ensure reliability.
- Updated documentation to reflect new features and usage instructions.

plugin/email/client.go

+package email
+
+import (
+	"crypto/tls"
+	"net/smtp"
+
+	"github.com/pkg/errors"
+)
+
+// Client represents an SMTP email client.
+type Client struct {
+	config *Config
+}
+
+// NewClient creates a new email client with the given configuration.
+func NewClient(config *Config) *Client {
+	return &Client{
+		config: config,
+	}
+}
+
+// validateConfig validates the client configuration.
+func (c *Client) validateConfig() error {
+	if c.config == nil {
+		return errors.New("email configuration is required")
+	}
+	return c.config.Validate()
+}
+
+// createAuth creates an SMTP auth mechanism if credentials are provided.
+func (c *Client) createAuth() smtp.Auth {
+	if c.config.SMTPUsername == "" && c.config.SMTPPassword == "" {
+		return nil
+	}
+	return smtp.PlainAuth("", c.config.SMTPUsername, c.config.SMTPPassword, c.config.SMTPHost)
+}
+
+// createTLSConfig creates a TLS configuration for secure connections.
+func (c *Client) createTLSConfig() *tls.Config {
+	return &tls.Config{
+		ServerName: c.config.SMTPHost,
+		MinVersion: tls.VersionTLS12,
+	}
+}
+
+// Send sends an email message via SMTP.
+func (c *Client) Send(message *Message) error {
+	// Validate configuration
+	if err := c.validateConfig(); err != nil {
+		return errors.Wrap(err, "invalid email configuration")
+	}
+
+	// Validate message
+	if message == nil {
+		return errors.New("message is required")
+	}
+	if err := message.Validate(); err != nil {
+		return errors.Wrap(err, "invalid email message")
+	}
+
+	// Format the message
+	body := message.Format(c.config.FromEmail, c.config.FromName)
+
+	// Get all recipients
+	recipients := message.GetAllRecipients()
+
+	// Create auth
+	auth := c.createAuth()
+
+	// Send based on encryption type
+	if c.config.UseSSL {
+		return c.sendWithSSL(auth, recipients, body)
+	}
+	return c.sendWithTLS(auth, recipients, body)
+}
+
+// sendWithTLS sends email using STARTTLS (port 587).
+func (c *Client) sendWithTLS(auth smtp.Auth, recipients []string, body string) error {
+	serverAddr := c.config.GetServerAddress()
+
+	if c.config.UseTLS {
+		// Use STARTTLS
+		return smtp.SendMail(serverAddr, auth, c.config.FromEmail, recipients, []byte(body))
+	}
+
+	// Send without encryption (not recommended)
+	return smtp.SendMail(serverAddr, auth, c.config.FromEmail, recipients, []byte(body))
+}
+
+// sendWithSSL sends email using SSL/TLS (port 465).
+func (c *Client) sendWithSSL(auth smtp.Auth, recipients []string, body string) error {
+	serverAddr := c.config.GetServerAddress()
+
+	// Create TLS connection
+	tlsConfig := c.createTLSConfig()
+	conn, err := tls.Dial("tcp", serverAddr, tlsConfig)
+	if err != nil {
+		return errors.Wrapf(err, "failed to connect to SMTP server with SSL: %s", serverAddr)
+	}
+	defer conn.Close()
+
+	// Create SMTP client
+	client, err := smtp.NewClient(conn, c.config.SMTPHost)
+	if err != nil {
+		return errors.Wrap(err, "failed to create SMTP client")
+	}
+	defer client.Quit()
+
+	// Authenticate
+	if auth != nil {
+		if err := client.Auth(auth); err != nil {
+			return errors.Wrap(err, "SMTP authentication failed")
+		}
+	}
+
+	// Set sender
+	if err := client.Mail(c.config.FromEmail); err != nil {
+		return errors.Wrap(err, "failed to set sender")
+	}
+
+	// Set recipients
+	for _, recipient := range recipients {
+		if err := client.Rcpt(recipient); err != nil {
+			return errors.Wrapf(err, "failed to set recipient: %s", recipient)
+		}
+	}
+
+	// Send message body
+	writer, err := client.Data()
+	if err != nil {
+		return errors.Wrap(err, "failed to send DATA command")
+	}
+
+	if _, err := writer.Write([]byte(body)); err != nil {
+		return errors.Wrap(err, "failed to write message body")
+	}
+
+	if err := writer.Close(); err != nil {
+		return errors.Wrap(err, "failed to close message writer")
+	}
+
+	return nil
+}

plugin/email/client_test.go

+package email
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNewClient(t *testing.T) {
+	config := &Config{
+		SMTPHost:     "smtp.example.com",
+		SMTPPort:     587,
+		SMTPUsername: "user@example.com",
+		SMTPPassword: "password",
+		FromEmail:    "noreply@example.com",
+		FromName:     "Test App",
+		UseTLS:       true,
+	}
+
+	client := NewClient(config)
+
+	assert.NotNil(t, client)
+	assert.Equal(t, config, client.config)
+}
+
+func TestClientValidateConfig(t *testing.T) {
+	tests := []struct {
+		name    string
+		config  *Config
+		wantErr bool
+	}{
+		{
+			name: "valid config",
+			config: &Config{
+				SMTPHost:  "smtp.example.com",
+				SMTPPort:  587,
+				FromEmail: "test@example.com",
+			},
+			wantErr: false,
+		},
+		{
+			name:    "nil config",
+			config:  nil,
+			wantErr: true,
+		},
+		{
+			name: "invalid config",
+			config: &Config{
+				SMTPHost: "",
+				SMTPPort: 587,
+			},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			client := NewClient(tt.config)
+			err := client.validateConfig()
+			if tt.wantErr {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}
+
+func TestClientSendValidation(t *testing.T) {
+	config := &Config{
+		SMTPHost:  "smtp.example.com",
+		SMTPPort:  587,
+		FromEmail: "test@example.com",
+	}
+	client := NewClient(config)
+
+	tests := []struct {
+		name    string
+		message *Message
+		wantErr bool
+	}{
+		{
+			name: "valid message",
+			message: &Message{
+				To:      []string{"recipient@example.com"},
+				Subject: "Test",
+				Body:    "Test body",
+			},
+			wantErr: false, // Will fail on actual send, but passes validation
+		},
+		{
+			name:    "nil message",
+			message: nil,
+			wantErr: true,
+		},
+		{
+			name: "invalid message",
+			message: &Message{
+				To:      []string{},
+				Subject: "Test",
+				Body:    "Test",
+			},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := client.Send(tt.message)
+			// We expect validation errors for invalid messages
+			// For valid messages, we'll get connection errors (which is expected in tests)
+			if tt.wantErr {
+				assert.Error(t, err)
+				// Should fail validation before attempting connection
+				assert.NotContains(t, err.Error(), "dial")
+			}
+			// Note: We don't assert NoError for valid messages because
+			// we don't have a real SMTP server in tests
+		})
+	}
+}

plugin/email/config.go

+package email
+
+import (
+	"fmt"
+
+	"github.com/pkg/errors"
+)
+
+// Config represents the SMTP configuration for email sending.
+// These settings should be provided by the self-hosted instance administrator.
+type Config struct {
+	// SMTPHost is the SMTP server hostname (e.g., "smtp.gmail.com")
+	SMTPHost string
+	// SMTPPort is the SMTP server port (common: 587 for TLS, 465 for SSL, 25 for unencrypted)
+	SMTPPort int
+	// SMTPUsername is the SMTP authentication username (usually the email address)
+	SMTPUsername string
+	// SMTPPassword is the SMTP authentication password or app-specific password
+	SMTPPassword string
+	// FromEmail is the email address that will appear in the "From" field
+	FromEmail string
+	// FromName is the display name that will appear in the "From" field
+	FromName string
+	// UseTLS enables STARTTLS encryption (recommended for port 587)
+	UseTLS bool
+	// UseSSL enables SSL/TLS encryption (for port 465)
+	UseSSL bool
+}
+
+// Validate checks if the configuration is valid.
+func (c *Config) Validate() error {
+	if c.SMTPHost == "" {
+		return errors.New("SMTP host is required")
+	}
+	if c.SMTPPort <= 0 || c.SMTPPort > 65535 {
+		return errors.New("SMTP port must be between 1 and 65535")
+	}
+	if c.FromEmail == "" {
+		return errors.New("from email is required")
+	}
+	return nil
+}
+
+// GetServerAddress returns the SMTP server address in the format "host:port".
+func (c *Config) GetServerAddress() string {
+	return fmt.Sprintf("%s:%d", c.SMTPHost, c.SMTPPort)
+}

plugin/email/config_test.go

+package email
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestConfigValidation(t *testing.T) {
+	tests := []struct {
+		name    string
+		config  *Config
+		wantErr bool
+	}{
+		{
+			name: "valid config",
+			config: &Config{
+				SMTPHost:     "smtp.gmail.com",
+				SMTPPort:     587,
+				SMTPUsername: "user@example.com",
+				SMTPPassword: "password",
+				FromEmail:    "noreply@example.com",
+				FromName:     "Memos",
+			},
+			wantErr: false,
+		},
+		{
+			name: "missing host",
+			config: &Config{
+				SMTPPort:     587,
+				SMTPUsername: "user@example.com",
+				SMTPPassword: "password",
+				FromEmail:    "noreply@example.com",
+			},
+			wantErr: true,
+		},
+		{
+			name: "invalid port",
+			config: &Config{
+				SMTPHost:     "smtp.gmail.com",
+				SMTPPort:     0,
+				SMTPUsername: "user@example.com",
+				SMTPPassword: "password",
+				FromEmail:    "noreply@example.com",
+			},
+			wantErr: true,
+		},
+		{
+			name: "missing from email",
+			config: &Config{
+				SMTPHost:     "smtp.gmail.com",
+				SMTPPort:     587,
+				SMTPUsername: "user@example.com",
+				SMTPPassword: "password",
+			},
+			wantErr: true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.config.Validate()
+			if tt.wantErr {
+				assert.Error(t, err)
+			} else {
+				assert.NoError(t, err)
+			}
+		})
+	}
+}
+
+func TestConfigGetServerAddress(t *testing.T) {
+	config := &Config{
+		SMTPHost: "smtp.gmail.com",
+		SMTPPort: 587,
+	}
+
+	expected := "smtp.gmail.com:587"
+	assert.Equal(t, expected, config.GetServerAddress())
+}

plugin/email/doc.go

+// Package email provides SMTP email sending functionality for self-hosted Memos instances.
+//
+// This package is designed for self-hosted environments where instance administrators
+// configure their own SMTP servers. It follows industry-standard patterns used by
+// platforms like GitHub, GitLab, and Discourse.
+//
+// # Configuration
+//
+// The package requires SMTP server configuration provided by the instance administrator:
+//
+//	config := &email.Config{
+//	    SMTPHost:     "smtp.gmail.com",
+//	    SMTPPort:     587,
+//	    SMTPUsername: "your-email@gmail.com",
+//	    SMTPPassword: "your-app-password",
+//	    FromEmail:    "noreply@yourdomain.com",
+//	    FromName:     "Memos Notifications",
+//	    UseTLS:       true,
+//	}
+//
+// # Common SMTP Settings
+//
+// Gmail (requires App Password):
+//   - Host: smtp.gmail.com
+//   - Port: 587 (TLS) or 465 (SSL)
+//   - Username: your-email@gmail.com
+//   - UseTLS: true (for port 587) or UseSSL: true (for port 465)
+//
+// SendGrid:
+//   - Host: smtp.sendgrid.net
+//   - Port: 587
+//   - Username: apikey
+//   - Password: your-sendgrid-api-key
+//   - UseTLS: true
+//
+// AWS SES:
+//   - Host: email-smtp.[region].amazonaws.com
+//   - Port: 587
+//   - Username: your-smtp-username
+//   - Password: your-smtp-password
+//   - UseTLS: true
+//
+// Mailgun:
+//   - Host: smtp.mailgun.org
+//   - Port: 587
+//   - Username: your-mailgun-smtp-username
+//   - Password: your-mailgun-smtp-password
+//   - UseTLS: true
+//
+// # Sending Email
+//
+// Synchronous (waits for completion):
+//
+//	message := &email.Message{
+//	    To:      []string{"user@example.com"},
+//	    Subject: "Welcome to Memos",
+//	    Body:    "Thank you for joining!",
+//	    IsHTML:  false,
+//	}
+//
+//	err := email.Send(config, message)
+//	if err != nil {
+//	    // Handle error
+//	}
+//
+// Asynchronous (returns immediately):
+//
+//	email.SendAsync(config, message)
+//	// Errors are logged but not returned
+//
+// # HTML Email
+//
+//	message := &email.Message{
+//	    To:      []string{"user@example.com"},
+//	    Subject: "Welcome!",
+//	    Body:    "<html><body><h1>Welcome to Memos!</h1></body></html>",
+//	    IsHTML:  true,
+//	}
+//
+// # Security Considerations
+//
+//   - Always use TLS (port 587) or SSL (port 465) for production
+//   - Store SMTP credentials securely (environment variables or secrets management)
+//   - Use app-specific passwords for services like Gmail
+//   - Validate and sanitize email content to prevent injection attacks
+//   - Rate limit email sending to prevent abuse
+//
+// # Error Handling
+//
+// The package returns descriptive errors for common issues:
+//   - Configuration validation errors (missing host, invalid port, etc.)
+//   - Message validation errors (missing recipients, subject, or body)
+//   - Connection errors (cannot reach SMTP server)
+//   - Authentication errors (invalid credentials)
+//   - SMTP protocol errors (recipient rejected, etc.)
+//
+// All errors are wrapped with context using github.com/pkg/errors for better debugging.
+package email

plugin/email/email.go

+package email
+
+import (
+	"log/slog"
+
+	"github.com/pkg/errors"
+)
+
+// Send sends an email synchronously.
+// Returns an error if the email fails to send.
+func Send(config *Config, message *Message) error {
+	if config == nil {
+		return errors.New("email configuration is required")
+	}
+	if message == nil {
+		return errors.New("email message is required")
+	}
+
+	client := NewClient(config)
+	return client.Send(message)
+}
+
+// SendAsync sends an email asynchronously.
+// It spawns a new goroutine to handle the sending and does not wait for the response.
+// Any errors are logged but not returned.
+func SendAsync(config *Config, message *Message) {
+	go func() {
+		if err := Send(config, message); err != nil {
+			// Since we're in a goroutine, we can only log the error
+			recipients := ""
+			if message != nil && len(message.To) > 0 {
+				recipients = message.To[0]
+				if len(message.To) > 1 {
+					recipients += " and others"
+				}
+			}
+
+			slog.Warn("Failed to send email asynchronously",
+				slog.String("recipients", recipients),
+				slog.Any("error", err))
+		}
+	}()
+}

plugin/email/email_test.go

+package email
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSend(t *testing.T) {
+	config := &Config{
+		SMTPHost:  "smtp.example.com",
+		SMTPPort:  587,
+		FromEmail: "test@example.com",
+	}
+
+	message := &Message{
+		To:      []string{"recipient@example.com"},
+		Subject: "Test",
+		Body:    "Test body",
+	}
+
+	// This will fail to connect (no real server), but should validate inputs
+	err := Send(config, message)
+	// We expect an error because there's no real SMTP server
+	// But it should be a connection error, not a validation error
+	assert.Error(t, err)
+	assert.Contains(t, err.Error(), "dial")
+}
+
+func TestSendValidation(t *testing.T) {
+	tests := []struct {
+		name    string
+		config  *Config
+		message *Message
+		wantErr bool
+		errMsg  string
+	}{
+		{
+			name:    "nil config",
+			config:  nil,
+			message: &Message{To: []string{"test@example.com"}, Subject: "Test", Body: "Test"},
+			wantErr: true,
+			errMsg:  "configuration is required",
+		},
+		{
+			name:    "nil message",
+			config:  &Config{SMTPHost: "smtp.example.com", SMTPPort: 587, FromEmail: "from@example.com"},
+			message: nil,
+			wantErr: true,
+			errMsg:  "message is required",
+		},
+		{
+			name: "invalid config",
+			config: &Config{
+				SMTPHost: "",
+				SMTPPort: 587,
+			},
+			message: &Message{To: []string{"test@example.com"}, Subject: "Test", Body: "Test"},
+			wantErr: true,
+			errMsg:  "invalid email configuration",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := Send(tt.config, tt.message)
+			if tt.wantErr {
+				assert.Error(t, err)
+				assert.Contains(t, err.Error(), tt.errMsg)
+			}
+		})
+	}
+}
+
+func TestSendAsync(t *testing.T) {
+	config := &Config{
+		SMTPHost:  "smtp.example.com",
+		SMTPPort:  587,
+		FromEmail: "test@example.com",
+	}
+
+	message := &Message{
+		To:      []string{"recipient@example.com"},
+		Subject: "Test Async",
+		Body:    "Test async body",
+	}
+
+	// SendAsync should not block
+	start := time.Now()
+	SendAsync(config, message)
+	duration := time.Since(start)
+
+	// Should return almost immediately (< 100ms)
+	assert.Less(t, duration, 100*time.Millisecond)
+
+	// Give goroutine time to start
+	time.Sleep(50 * time.Millisecond)
+}
+
+func TestSendAsyncConcurrent(t *testing.T) {
+	config := &Config{
+		SMTPHost:  "smtp.example.com",
+		SMTPPort:  587,
+		FromEmail: "test@example.com",
+	}
+
+	// Send multiple emails concurrently
+	var wg sync.WaitGroup
+	count := 5
+
+	for i := 0; i < count; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			message := &Message{
+				To:      []string{"recipient@example.com"},
+				Subject: "Concurrent Test",
+				Body:    "Test body",
+			}
+			SendAsync(config, message)
+		}()
+	}
+
+	// Should complete without deadlock
+	done := make(chan bool)
+	go func() {
+		wg.Wait()
+		done <- true
+	}()
+
+	select {
+	case <-done:
+		// Success
+	case <-time.After(1 * time.Second):
+		t.Fatal("SendAsync calls did not complete in time")
+	}
+}

plugin/email/message.go

+package email
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"time"
+)
+
+// Message represents an email message to be sent.
+type Message struct {
+	To      []string // Required: recipient email addresses
+	Cc      []string // Optional: carbon copy recipients
+	Bcc     []string // Optional: blind carbon copy recipients
+	Subject string   // Required: email subject
+	Body    string   // Required: email body content
+	IsHTML  bool     // Whether the body is HTML (default: false for plain text)
+	ReplyTo string   // Optional: reply-to address
+}
+
+// Validate checks that the message has all required fields.
+func (m *Message) Validate() error {
+	if len(m.To) == 0 {
+		return errors.New("at least one recipient is required")
+	}
+	if m.Subject == "" {
+		return errors.New("subject is required")
+	}
+	if m.Body == "" {
+		return errors.New("body is required")
+	}
+	return nil
+}
+
+// Format creates an RFC 5322 formatted email message.
+func (m *Message) Format(fromEmail, fromName string) string {
+	var sb strings.Builder
+
+	// From header
+	if fromName != "" {
+		sb.WriteString(fmt.Sprintf("From: %s <%s>\r\n", fromName, fromEmail))
+	} else {
+		sb.WriteString(fmt.Sprintf("From: %s\r\n", fromEmail))
+	}
+
+	// To header
+	sb.WriteString(fmt.Sprintf("To: %s\r\n", strings.Join(m.To, ", ")))
+
+	// Cc header (optional)
+	if len(m.Cc) > 0 {
+		sb.WriteString(fmt.Sprintf("Cc: %s\r\n", strings.Join(m.Cc, ", ")))
+	}
+
+	// Reply-To header (optional)
+	if m.ReplyTo != "" {
+		sb.WriteString(fmt.Sprintf("Reply-To: %s\r\n", m.ReplyTo))
+	}
+
+	// Subject header
+	sb.WriteString(fmt.Sprintf("Subject: %s\r\n", m.Subject))
+
+	// Date header (RFC 5322 format)
+	sb.WriteString(fmt.Sprintf("Date: %s\r\n", time.Now().Format(time.RFC1123Z)))
+
+	// MIME headers
+	sb.WriteString("MIME-Version: 1.0\r\n")
+
+	// Content-Type header
+	if m.IsHTML {
+		sb.WriteString("Content-Type: text/html; charset=utf-8\r\n")
+	} else {
+		sb.WriteString("Content-Type: text/plain; charset=utf-8\r\n")
+	}
+
+	// Empty line separating headers from body
+	sb.WriteString("\r\n")
+
+	// Body
+	sb.WriteString(m.Body)
+
+	return sb.String()
+}
+
+// GetAllRecipients returns all recipients (To, Cc, Bcc) as a single slice.
+func (m *Message) GetAllRecipients() []string {
+	var recipients []string
+	recipients = append(recipients, m.To...)
+	recipients = append(recipients, m.Cc...)
+	recipients = append(recipients, m.Bcc...)
+	return recipients
+}

plugin/email/message_test.go

+package email
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestMessageValidation(t *testing.T) {
+	tests := []struct {
+		name    string
+		msg     Message
+		wantErr bool
+	}{
+		{
+			name: "valid message",
+			msg: Message{
+				To:      []string{"user@example.com"},
+				Subject: "Test Subject",
+				Body:    "Test Body",
+			},
+			wantErr: false,
+		},
+		{
+			name: "no recipients",
+			msg: Message{
+				To:      []string{},
+				Subject: "Test Subject",
+				Body:    "Test Body",
+			},
+			wantErr: true,
+		},
+		{
+			name: "no subject",
+			msg: Message{
+				To:      []string{"user@example.com"},
+				Subject: "",
+				Body:    "Test Body",
+			},
+			wantErr: true,
+		},
+		{
+			name: "no body",
+			msg: Message{
+				To:      []string{"user@example.com"},
+				Subject: "Test Subject",
+				Body:    "",
+			},
+			wantErr: true,
+		},
+		{
+			name: "multiple recipients",
+			msg: Message{
+				To:      []string{"user1@example.com", "user2@example.com"},
+				Cc:      []string{"cc@example.com"},
+				Subject: "Test Subject",
+				Body:    "Test Body",
+			},
+			wantErr: false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := tt.msg.Validate()
+			if (err != nil) != tt.wantErr {
+				t.Errorf("Validate() error = %v, wantErr %v", err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestMessageFormatPlainText(t *testing.T) {
+	msg := Message{
+		To:      []string{"user@example.com"},
+		Subject: "Test Subject",
+		Body:    "Test Body",
+		IsHTML:  false,
+	}
+
+	formatted := msg.Format("sender@example.com", "Sender Name")
+
+	// Check required headers
+	if !strings.Contains(formatted, "From: Sender Name <sender@example.com>") {
+		t.Error("Missing or incorrect From header")
+	}
+	if !strings.Contains(formatted, "To: user@example.com") {
+		t.Error("Missing or incorrect To header")
+	}
+	if !strings.Contains(formatted, "Subject: Test Subject") {
+		t.Error("Missing or incorrect Subject header")
+	}
+	if !strings.Contains(formatted, "Content-Type: text/plain; charset=utf-8") {
+		t.Error("Missing or incorrect Content-Type header for plain text")
+	}
+	if !strings.Contains(formatted, "Test Body") {
+		t.Error("Missing message body")
+	}
+}
+
+func TestMessageFormatHTML(t *testing.T) {
+	msg := Message{
+		To:      []string{"user@example.com"},
+		Subject: "Test Subject",
+		Body:    "<html><body>Test Body</body></html>",
+		IsHTML:  true,
+	}
+
+	formatted := msg.Format("sender@example.com", "Sender Name")
+
+	// Check HTML content-type
+	if !strings.Contains(formatted, "Content-Type: text/html; charset=utf-8") {
+		t.Error("Missing or incorrect Content-Type header for HTML")
+	}
+	if !strings.Contains(formatted, "<html><body>Test Body</body></html>") {
+		t.Error("Missing HTML body")
+	}
+}
+
+func TestMessageFormatMultipleRecipients(t *testing.T) {
+	msg := Message{
+		To:      []string{"user1@example.com", "user2@example.com"},
+		Cc:      []string{"cc1@example.com", "cc2@example.com"},
+		Bcc:     []string{"bcc@example.com"},
+		Subject: "Test Subject",
+		Body:    "Test Body",
+		ReplyTo: "reply@example.com",
+	}
+
+	formatted := msg.Format("sender@example.com", "Sender Name")
+
+	// Check To header formatting
+	if !strings.Contains(formatted, "To: user1@example.com, user2@example.com") {
+		t.Error("Missing or incorrect To header with multiple recipients")
+	}
+	// Check Cc header formatting
+	if !strings.Contains(formatted, "Cc: cc1@example.com, cc2@example.com") {
+		t.Error("Missing or incorrect Cc header")
+	}
+	// Bcc should NOT appear in the formatted message
+	if strings.Contains(formatted, "Bcc:") {
+		t.Error("Bcc header should not appear in formatted message")
+	}
+	// Check Reply-To header
+	if !strings.Contains(formatted, "Reply-To: reply@example.com") {
+		t.Error("Missing or incorrect Reply-To header")
+	}
+}
+
+func TestGetAllRecipients(t *testing.T) {
+	msg := Message{
+		To:  []string{"user1@example.com", "user2@example.com"},
+		Cc:  []string{"cc@example.com"},
+		Bcc: []string{"bcc@example.com"},
+	}
+
+	recipients := msg.GetAllRecipients()
+
+	// Should have all 4 recipients
+	if len(recipients) != 4 {
+		t.Errorf("GetAllRecipients() returned %d recipients, want 4", len(recipients))
+	}
+
+	// Check all recipients are present
+	expectedRecipients := map[string]bool{
+		"user1@example.com": true,
+		"user2@example.com": true,
+		"cc@example.com":    true,
+		"bcc@example.com":   true,
+	}
+
+	for _, recipient := range recipients {
+		if !expectedRecipients[recipient] {
+			t.Errorf("Unexpected recipient: %s", recipient)
+		}
+		delete(expectedRecipients, recipient)
+	}
+
+	if len(expectedRecipients) > 0 {
+		t.Error("Not all expected recipients were returned")
+	}
+}