Feature: Allow rejected SMTP recipients to be silently dropped (#549)

cmd/root.go

@@ -126,6 +126,7 @@ func init() {
 	rootCmd.Flags().BoolVar(&config.SMTPStrictRFCHeaders, "smtp-strict-rfc-headers", config.SMTPStrictRFCHeaders, "Return SMTP error if message headers contain <CR><CR><LF>")
 	rootCmd.Flags().IntVar(&config.SMTPMaxRecipients, "smtp-max-recipients", config.SMTPMaxRecipients, "Maximum SMTP recipients allowed")
 	rootCmd.Flags().StringVar(&config.SMTPAllowedRecipients, "smtp-allowed-recipients", config.SMTPAllowedRecipients, "Only allow SMTP recipients matching a regular expression (default allow all)")
+	rootCmd.Flags().BoolVar(&config.SMTPSilentlyDropRejectedRecipients, "smtp-silently-drop-rejected-recipients", config.SMTPSilentlyDropRejectedRecipients, "Accept emails to rejected recipients with 2xx response but silently drop them")
 	rootCmd.Flags().BoolVar(&smtpd.DisableReverseDNS, "smtp-disable-rdns", smtpd.DisableReverseDNS, "Disable SMTP reverse DNS lookups")
 
 	// SMTP relay
@@ -301,6 +302,9 @@ func initConfigFromEnv() {
 	if len(os.Getenv("MP_SMTP_ALLOWED_RECIPIENTS")) > 0 {
 		config.SMTPAllowedRecipients = os.Getenv("MP_SMTP_ALLOWED_RECIPIENTS")
 	}
+	if getEnabledFromEnv("MP_SMTP_SILENTLY_DROP_REJECTED_RECIPIENTS") {
+		config.SMTPSilentlyDropRejectedRecipients = true
+	}
 	if getEnabledFromEnv("MP_SMTP_DISABLE_RDNS") {
 		smtpd.DisableReverseDNS = true
 	}

config/config.go

@@ -181,6 +181,9 @@ var (
 	// SMTPAllowedRecipientsRegexp is the compiled version of SMTPAllowedRecipients
 	SMTPAllowedRecipientsRegexp *regexp.Regexp
 
+	// SMTPSilentlyDropRejectedRecipients if true, will accept emails to rejected recipients with 2xx response but silently drop them
+	SMTPSilentlyDropRejectedRecipients bool
+
 	// POP3Listen address - if set then Mailpit will start the POP3 server and listen on this address
 	POP3Listen = "[::]:1110"
 

internal/smtpd/main.go

@@ -194,15 +194,16 @@ func listenAndServe(addr string, handler MsgIDHandler, authHandler AuthHandler)
 
 	Debug = true // to enable Mailpit logging
 	srv := &Server{
-		Addr:              addr,
-		MsgIDHandler:      handler,
-		HandlerRcpt:       handlerRcpt,
-		AppName:           "Mailpit",
-		Hostname:          "",
-		AuthHandler:       nil,
-		AuthRequired:      false,
-		MaxRecipients:     config.SMTPMaxRecipients,
-		DisableReverseDNS: DisableReverseDNS,
+		Addr:                           addr,
+		MsgIDHandler:                   handler,
+		HandlerRcpt:                    handlerRcpt,
+		AppName:                        "Mailpit",
+		Hostname:                       "",
+		AuthHandler:                    nil,
+		AuthRequired:                   false,
+		MaxRecipients:                  config.SMTPMaxRecipients,
+		SilentlyDropRejectedRecipients: config.SMTPSilentlyDropRejectedRecipients,
+		DisableReverseDNS:              DisableReverseDNS,
 		LogRead: func(remoteIP, verb, line string) {
 			logger.Log().Debugf("[smtpd] %s (%s) %s", verbLogTranslator(verb), remoteIP, line)
 		},

internal/smtpd/smtpd.go

@@ -92,26 +92,27 @@ type LogFunc func(remoteIP, verb, line string)
 
 // Server is an SMTP server.
 type Server struct {
-	Addr              string // TCP address to listen on, defaults to ":25" (all addresses, port 25) if empty
-	AppName           string
-	AuthHandler       AuthHandler
-	AuthMechs         map[string]bool // Override list of allowed authentication mechanisms. Currently supported: LOGIN, PLAIN, CRAM-MD5. Enabling LOGIN and PLAIN will reduce RFC 4954 compliance.
-	AuthRequired      bool            // Require authentication for every command except AUTH, EHLO, HELO, NOOP, RSET or QUIT as per RFC 4954. Ignored if AuthHandler is not configured.
-	DisableReverseDNS bool            // Disable reverse DNS lookups, enforces "unknown" hostname
-	Handler           Handler
-	HandlerRcpt       HandlerRcpt
-	Hostname          string
-	LogRead           LogFunc
-	LogWrite          LogFunc
-	MaxSize           int // Maximum message size allowed, in bytes
-	MaxRecipients     int // Maximum number of recipients, defaults to 100.
-	MsgIDHandler      MsgIDHandler
-	Timeout           time.Duration
-	TLSConfig         *tls.Config
-	TLSListener       bool        // Listen for incoming TLS connections only (not recommended as it may reduce compatibility). Ignored if TLS is not configured.
-	TLSRequired       bool        // Require TLS for every command except NOOP, EHLO, STARTTLS, or QUIT as per RFC 3207. Ignored if TLS is not configured.
-	Protocol          string      // Default tcp, supports unix
-	SocketPerm        fs.FileMode // if using Unix socket, socket permissions
+	Addr                           string // TCP address to listen on, defaults to ":25" (all addresses, port 25) if empty
+	AppName                        string
+	AuthHandler                    AuthHandler
+	AuthMechs                      map[string]bool // Override list of allowed authentication mechanisms. Currently supported: LOGIN, PLAIN, CRAM-MD5. Enabling LOGIN and PLAIN will reduce RFC 4954 compliance.
+	AuthRequired                   bool            // Require authentication for every command except AUTH, EHLO, HELO, NOOP, RSET or QUIT as per RFC 4954. Ignored if AuthHandler is not configured.
+	DisableReverseDNS              bool            // Disable reverse DNS lookups, enforces "unknown" hostname
+	Handler                        Handler
+	HandlerRcpt                    HandlerRcpt
+	Hostname                       string
+	LogRead                        LogFunc
+	LogWrite                       LogFunc
+	MaxSize                        int // Maximum message size allowed, in bytes
+	MaxRecipients                  int // Maximum number of recipients, defaults to 100.
+	MsgIDHandler                   MsgIDHandler
+	SilentlyDropRejectedRecipients bool // Accept emails to rejected recipients with 2xx response but silently drop them
+	Timeout                        time.Duration
+	TLSConfig                      *tls.Config
+	TLSListener                    bool        // Listen for incoming TLS connections only (not recommended as it may reduce compatibility). Ignored if TLS is not configured.
+	TLSRequired                    bool        // Require TLS for every command except NOOP, EHLO, STARTTLS, or QUIT as per RFC 3207. Ignored if TLS is not configured.
+	Protocol                       string      // Default tcp, supports unix
+	SocketPerm                     fs.FileMode // if using Unix socket, socket permissions
 
 	inShutdown   int32 // server was closed or shutdown
 	openSessions int32 // count of open sessions
@@ -363,6 +364,7 @@ func (s *session) serve() {
 	var from string
 	var gotFrom bool
 	var to []string
+	var hasRejectedRecipients bool
 	var buffer bytes.Buffer
 
 	// RFC 5321 specifies support for minimum of 100 recipients is required.
@@ -397,6 +399,7 @@ loop:
 			from = ""
 			gotFrom = false
 			to = nil
+			hasRejectedRecipients = false
 			buffer.Reset()
 		case "EHLO":
 			s.remoteName = args
@@ -406,6 +409,7 @@ loop:
 			from = ""
 			gotFrom = false
 			to = nil
+			hasRejectedRecipients = false
 			buffer.Reset()
 		case "MAIL":
 			if s.srv.TLSConfig != nil && s.srv.TLSRequired && !s.tls {
@@ -457,6 +461,7 @@ loop:
 			}
 
 			to = nil
+			hasRejectedRecipients = false
 			buffer.Reset()
 		case "RCPT":
 			if s.srv.TLSConfig != nil && s.srv.TLSRequired && !s.tls {
@@ -492,6 +497,9 @@ loop:
 					if accept {
 						to = append(to, match[1])
 						s.writef("250 2.1.5 Ok")
+					} else if s.srv.SilentlyDropRejectedRecipients {
+						hasRejectedRecipients = true
+						s.writef("250 2.1.5 Ok")
 					} else {
 						s.writef("550 5.1.0 Requested action not taken: mailbox unavailable")
 					}
@@ -506,7 +514,8 @@ loop:
 				s.writef("530 5.7.0 Authentication required")
 				break
 			}
-			if !gotFrom || len(to) == 0 {
+			hasRecipients := len(to) > 0 || hasRejectedRecipients
+			if !gotFrom || !hasRecipients {
 				s.writef("503 5.5.1 Bad sequence of commands (MAIL & RCPT required before DATA)")
 				break
 			}
@@ -536,11 +545,13 @@ loop:
 
 			// Create Received header & write message body into buffer.
 			buffer.Reset()
-			buffer.Write(s.makeHeaders(to))
+			if len(to) > 0 {
+				buffer.Write(s.makeHeaders(to))
+			}
 			buffer.Write(data)
 
-			// Pass mail on to handler.
-			if s.srv.Handler != nil {
+			// Pass mail on to handler only if there are valid recipients.
+			if len(to) > 0 && s.srv.Handler != nil {
 				err := s.srv.Handler(s.conn.RemoteAddr(), from, to, buffer.Bytes())
 				if err != nil {
 					checkErrFormat := regexp.MustCompile(`^([2-5][0-9]{2})[\s\-](.+)$`)
@@ -552,7 +563,7 @@ loop:
 					break
 				}
 				s.writef("250 2.0.0 Ok: queued")
-			} else if s.srv.MsgIDHandler != nil {
+			} else if len(to) > 0 && s.srv.MsgIDHandler != nil {
 				msgID, err := s.srv.MsgIDHandler(s.conn.RemoteAddr(), from, to, buffer.Bytes(), s.username)
 				if err != nil {
 					checkErrFormat := regexp.MustCompile(`^([2-5][0-9]{2})[\s\-](.+)$`)
@@ -570,6 +581,13 @@ loop:
 					s.writef("250 2.0.0 Ok: queued")
 				}
 			} else {
+				if hasRejectedRecipients && Debug {
+					if s.srv.LogWrite != nil {
+						s.srv.LogWrite(s.remoteIP, "DEBUG", "Message from sender silently dropped (rejected recipients)")
+					} else {
+						log.Printf("%s DEBUG Message from sender silently dropped (rejected recipients)", s.remoteIP)
+					}
+				}
 				s.writef("250 2.0.0 Ok: queued")
 			}
 
@@ -577,6 +595,7 @@ loop:
 			from = ""
 			gotFrom = false
 			to = nil
+			hasRejectedRecipients = false
 			buffer.Reset()
 		case "QUIT":
 			s.writef("221 2.0.0 %s %s ESMTP Service closing transmission channel", s.srv.Hostname, s.srv.AppName)
@@ -590,6 +609,7 @@ loop:
 			from = ""
 			gotFrom = false
 			to = nil
+			hasRejectedRecipients = false
 			buffer.Reset()
 		case "NOOP":
 			s.writef("250 2.0.0 Ok")
@@ -666,6 +686,7 @@ loop:
 			from = ""
 			gotFrom = false
 			to = nil
+			hasRejectedRecipients = false
 			buffer.Reset()
 		case "AUTH":
 			if s.srv.TLSConfig != nil && s.srv.TLSRequired && !s.tls {

internal/smtpd/smtpd_test.go

@@ -1598,3 +1598,172 @@ func TestCmdShutdown(t *testing.T) {
 
 	_ = conn.Close()
 }
+
+type mockDropRejectedHandler struct {
+	handlerCalled int
+	lastFrom      string
+	lastTo        []string
+	msgIDCalled   int
+	lastMsgIDFrom string
+	lastMsgIDTo   []string
+}
+
+func (m *mockDropRejectedHandler) handler(remoteAddr net.Addr, from string, to []string, data []byte) error {
+	m.handlerCalled++
+	m.lastFrom = from
+	m.lastTo = append([]string{}, to...) // copy slice
+	return nil
+}
+
+func (m *mockDropRejectedHandler) msgIDHandler(remoteAddr net.Addr, from string, to []string, data []byte, username *string) (string, error) {
+	m.msgIDCalled++
+	m.lastMsgIDFrom = from
+	m.lastMsgIDTo = append([]string{}, to...) // copy slice
+	return "test-message-id", nil
+}
+
+// Test the SilentlyDropRejectedRecipients option
+func TestSilentlyDropRejectedRecipients(t *testing.T) {
+	tests := []struct {
+		name                           string
+		silentlyDropRejectedRecipients bool
+		handlerRcpt                    func(net.Addr, string, string) bool
+		rcptCommands                   []struct{ addr, expectedCode string }
+		expectedHandlerCalls           int
+		expectedHandlerRecipients      []string
+		useMsgIDHandler                bool
+	}{
+		{
+			name:                           "Disabled_DefaultBehavior",
+			silentlyDropRejectedRecipients: false,
+			handlerRcpt: func(remoteAddr net.Addr, from string, to string) bool {
+				return !strings.HasSuffix(to, "@rejected.com")
+			},
+			rcptCommands: []struct{ addr, expectedCode string }{
+				{"valid@example.com", "250"},
+				{"invalid@rejected.com", "550"},
+			},
+			expectedHandlerCalls:      1,
+			expectedHandlerRecipients: []string{"valid@example.com"},
+		},
+		{
+			name:                           "Enabled_MixedRecipients",
+			silentlyDropRejectedRecipients: true,
+			handlerRcpt: func(remoteAddr net.Addr, from string, to string) bool {
+				return !strings.HasSuffix(to, "@rejected.com")
+			},
+			rcptCommands: []struct{ addr, expectedCode string }{
+				{"valid1@example.com", "250"},
+				{"valid2@example.com", "250"},
+				{"invalid1@rejected.com", "250"}, // Now accepted but dropped
+				{"invalid2@rejected.com", "250"}, // Now accepted but dropped
+			},
+			expectedHandlerCalls:      1,
+			expectedHandlerRecipients: []string{"valid1@example.com", "valid2@example.com"},
+		},
+		{
+			name:                           "Enabled_AllRejected",
+			silentlyDropRejectedRecipients: true,
+			handlerRcpt: func(remoteAddr net.Addr, from string, to string) bool {
+				return false // Reject all
+			},
+			rcptCommands: []struct{ addr, expectedCode string }{
+				{"test1@example.com", "250"}, // Accepted but dropped
+				{"test2@example.com", "250"}, // Accepted but dropped
+			},
+			expectedHandlerCalls:      0, // No handler calls since all rejected
+			expectedHandlerRecipients: nil,
+		},
+		{
+			name:                           "Enabled_OnlyValid",
+			silentlyDropRejectedRecipients: true,
+			handlerRcpt: func(remoteAddr net.Addr, from string, to string) bool {
+				return strings.HasSuffix(to, "@valid.com")
+			},
+			rcptCommands: []struct{ addr, expectedCode string }{
+				{"user1@valid.com", "250"},
+				{"user2@valid.com", "250"},
+				{"user3@valid.com", "250"},
+			},
+			expectedHandlerCalls:      1,
+			expectedHandlerRecipients: []string{"user1@valid.com", "user2@valid.com", "user3@valid.com"},
+		},
+		{
+			name:                           "Enabled_WithMsgIDHandler",
+			silentlyDropRejectedRecipients: true,
+			handlerRcpt: func(remoteAddr net.Addr, from string, to string) bool {
+				return !strings.HasSuffix(to, "@rejected.com")
+			},
+			rcptCommands: []struct{ addr, expectedCode string }{
+				{"valid@example.com", "250"},
+				{"invalid@rejected.com", "250"}, // Accepted but dropped
+			},
+			expectedHandlerCalls:      1,
+			expectedHandlerRecipients: []string{"valid@example.com"},
+			useMsgIDHandler:           true,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			mock := &mockDropRejectedHandler{}
+
+			server := &Server{
+				Hostname:                       "mail.example.com",
+				AppName:                        "TestMail",
+				MaxRecipients:                  100,
+				HandlerRcpt:                    tt.handlerRcpt,
+				SilentlyDropRejectedRecipients: tt.silentlyDropRejectedRecipients,
+			}
+
+			if tt.useMsgIDHandler {
+				server.MsgIDHandler = mock.msgIDHandler
+			} else {
+				server.Handler = mock.handler
+			}
+
+			conn := newConn(t, server)
+			defer func() { _ = conn.Close() }()
+
+			cmdCode(t, conn, "HELO host.example.com", "250")
+			cmdCode(t, conn, "MAIL FROM:<sender@example.com>", "250")
+
+			// Send RCPT commands
+			for _, rcpt := range tt.rcptCommands {
+				cmdCode(t, conn, "RCPT TO:<"+rcpt.addr+">", rcpt.expectedCode)
+			}
+
+			// Send DATA
+			cmdCode(t, conn, "DATA", "354")
+			cmdCode(t, conn, "Subject: Test\r\n\r\nTest message\r\n.", "250")
+			cmdCode(t, conn, "QUIT", "221")
+
+			// Verify handler calls
+			if tt.useMsgIDHandler {
+				if mock.msgIDCalled != tt.expectedHandlerCalls {
+					t.Errorf("Expected %d MsgIDHandler calls, got %d", tt.expectedHandlerCalls, mock.msgIDCalled)
+				}
+				if tt.expectedHandlerCalls > 0 {
+					if mock.lastMsgIDFrom != "sender@example.com" {
+						t.Errorf("Expected from 'sender@example.com', got '%s'", mock.lastMsgIDFrom)
+					}
+					if !reflect.DeepEqual(mock.lastMsgIDTo, tt.expectedHandlerRecipients) {
+						t.Errorf("Expected recipients %v, got %v", tt.expectedHandlerRecipients, mock.lastMsgIDTo)
+					}
+				}
+			} else {
+				if mock.handlerCalled != tt.expectedHandlerCalls {
+					t.Errorf("Expected %d handler calls, got %d", tt.expectedHandlerCalls, mock.handlerCalled)
+				}
+				if tt.expectedHandlerCalls > 0 {
+					if mock.lastFrom != "sender@example.com" {
+						t.Errorf("Expected from 'sender@example.com', got '%s'", mock.lastFrom)
+					}
+					if !reflect.DeepEqual(mock.lastTo, tt.expectedHandlerRecipients) {
+						t.Errorf("Expected recipients %v, got %v", tt.expectedHandlerRecipients, mock.lastTo)
+					}
+				}
+			}
+		})
+	}
+}