Feature: Add option to only allow SMTP recipients matching a regular expression (disable open-relay behaviour #219)

cmd/root.go

@@ -103,6 +103,7 @@ func init() {
 	rootCmd.Flags().BoolVar(&config.SMTPAuthAllowInsecure, "smtp-auth-allow-insecure", config.SMTPAuthAllowInsecure, "Enable insecure PLAIN & LOGIN authentication")
 	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().StringVar(&config.SMTPRelayConfigFile, "smtp-relay-config", config.SMTPRelayConfigFile, "SMTP configuration file to allow releasing messages")
 	rootCmd.Flags().BoolVar(&config.SMTPRelayAllIncoming, "smtp-relay-all", config.SMTPRelayAllIncoming, "Relay all incoming messages via external SMTP server (caution!)")
@@ -170,6 +171,9 @@ func initConfigFromEnv() {
 	if len(os.Getenv("MP_SMTP_MAX_RECIPIENTS")) > 0 {
 		config.SMTPMaxRecipients, _ = strconv.Atoi(os.Getenv("MP_SMTP_MAX_RECIPIENTS"))
 	}
+	if len(os.Getenv("MP_SMTP_ALLOWED_RECIPIENTS")) > 0 {
+		config.SMTPAllowedRecipients = os.Getenv("MP_SMTP_ALLOWED_RECIPIENTS")
+	}
 
 	// Relay server config
 	config.SMTPRelayConfigFile = os.Getenv("MP_SMTP_RELAY_CONFIG")

config/config.go

@@ -93,6 +93,12 @@ var (
 	// @see https://github.com/axllent/mailpit/issues/87 & https://github.com/axllent/mailpit/issues/153
 	SMTPStrictRFCHeaders bool
 
+	// SMTPAllowedRecipients if set, will only accept recipients matching this regular expression
+	SMTPAllowedRecipients string
+
+	// SMTPAllowedRecipientsRegexp is the compiled version of SMTPAllowedRecipients
+	SMTPAllowedRecipientsRegexp *regexp.Regexp
+
 	// ReleaseEnabled is whether message releases are enabled, requires a valid SMTPRelayConfigFile
 	ReleaseEnabled = false
 
@@ -262,6 +268,16 @@ func VerifyConfig() error {
 		}
 	}
 
+	if SMTPAllowedRecipients != "" {
+		restrictRegexp, err := regexp.Compile(SMTPAllowedRecipients)
+		if err != nil {
+			return fmt.Errorf("Failed to compile smtp-allowed-recipients regexp: %s", err.Error())
+		}
+
+		SMTPAllowedRecipientsRegexp = restrictRegexp
+		logger.Log().Infof("[smtp] only allowing recipients matching the following regexp: %s", SMTPAllowedRecipients)
+	}
+
 	if err := parseRelayConfig(SMTPRelayConfigFile); err != nil {
 		return err
 	}
@@ -335,11 +351,11 @@ func parseRelayConfig(c string) error {
 
 	if SMTPRelayConfig.RecipientAllowlist != "" {
 		if err != nil {
-			return fmt.Errorf("failed to compile recipient allowlist regexp: %e", err)
+			return fmt.Errorf("Failed to compile relay recipient allowlist regexp: %s", err.Error())
 		}
 
 		SMTPRelayConfig.RecipientAllowlistRegexp = allowlistRegexp
-		logger.Log().Infof("[smtp] recipient allowlist is active with the following regexp: %s", SMTPRelayConfig.RecipientAllowlist)
+		logger.Log().Infof("[smtp] relay recipient allowlist is active with the following regexp: %s", SMTPRelayConfig.RecipientAllowlist)
 
 	}
 

internal/stats/stats.go

@@ -21,9 +21,9 @@ var (
 
 	mu sync.RWMutex
 
-	smtpReceived     int
-	smtpReceivedSize int
-	smtpErrors       int
+	smtpAccepted     int
+	smtpAcceptedSize int
+	smtpRejected     int
 	smtpIgnored      int
 )
 
@@ -52,13 +52,13 @@ type AppInformation struct {
 		Memory uint64
 		// Messages deleted
 		MessagesDeleted int
-		// SMTP messages received via since run
-		SMTPReceived int
-		// Total size in bytes of received messages since run
-		SMTPReceivedSize int
-		// SMTP errors since run
-		SMTPErrors int
-		// SMTP messages ignored since run (duplicate IDs)
+		// SMTP accepted messages since run
+		SMTPAccepted int
+		// Total size in bytes of accepted messages since run
+		SMTPAcceptedSize int
+		// SMTP rejected messages since run
+		SMTPRejected int
+		// SMTP ignored messages since run (duplicate IDs)
 		SMTPIgnored int
 	}
 }
@@ -75,9 +75,9 @@ func Load() AppInformation {
 
 	info.RuntimeStats.Uptime = int(time.Since(startedAt).Seconds())
 	info.RuntimeStats.MessagesDeleted = storage.StatsDeleted
-	info.RuntimeStats.SMTPReceived = smtpReceived
-	info.RuntimeStats.SMTPReceivedSize = smtpReceivedSize
-	info.RuntimeStats.SMTPErrors = smtpErrors
+	info.RuntimeStats.SMTPAccepted = smtpAccepted
+	info.RuntimeStats.SMTPAcceptedSize = smtpAcceptedSize
+	info.RuntimeStats.SMTPRejected = smtpRejected
 	info.RuntimeStats.SMTPIgnored = smtpIgnored
 
 	if latestVersionCache != "" {
@@ -116,18 +116,18 @@ func Track() {
 	startedAt = time.Now()
 }
 
-// LogSMTPReceived logs a successfully SMTP transaction
-func LogSMTPReceived(size int) {
+// LogSMTPAccepted logs a successful SMTP transaction
+func LogSMTPAccepted(size int) {
 	mu.Lock()
-	smtpReceived = smtpReceived + 1
-	smtpReceivedSize = smtpReceivedSize + size
+	smtpAccepted = smtpAccepted + 1
+	smtpAcceptedSize = smtpAcceptedSize + size
 	mu.Unlock()
 }
 
-// LogSMTPError logs a failed SMTP transaction
-func LogSMTPError() {
+// LogSMTPRejected logs a rejected SMTP transaction
+func LogSMTPRejected() {
 	mu.Lock()
-	smtpErrors = smtpErrors + 1
+	smtpRejected = smtpRejected + 1
 	mu.Unlock()
 }
 

server/smtpd/smtpd.go

@@ -28,7 +28,7 @@ func mailHandler(origin net.Addr, from string, to []string, data []byte) error {
 	msg, err := mail.ReadMessage(bytes.NewReader(data))
 	if err != nil {
 		logger.Log().Errorf("[smtpd] error parsing message: %s", err.Error())
-		stats.LogSMTPError()
+		stats.LogSMTPRejected()
 		return err
 	}
 
@@ -121,11 +121,10 @@ func mailHandler(origin net.Addr, from string, to []string, data []byte) error {
 	_, err = storage.Store(&data)
 	if err != nil {
 		logger.Log().Errorf("[db] error storing message: %s", err.Error())
-		stats.LogSMTPError()
 		return err
 	}
 
-	stats.LogSMTPReceived(len(data))
+	stats.LogSMTPAccepted(len(data))
 
 	data = nil // avoid memory leaks
 
@@ -153,6 +152,22 @@ func authHandlerAny(remoteAddr net.Addr, mechanism string, username []byte, _ []
 	return true, nil
 }
 
+// HandlerRcpt used to optionally restrict recipients based on `--smtp-allowed-recipients`
+func handlerRcpt(remoteAddr net.Addr, from string, to string) bool {
+	if config.SMTPAllowedRecipientsRegexp == nil {
+		return true
+	}
+
+	result := config.SMTPAllowedRecipientsRegexp.MatchString(to)
+
+	if !result {
+		logger.Log().Warnf("[smtpd] rejected message to %s from %s (%s)", to, from, cleanIP(remoteAddr))
+		stats.LogSMTPRejected()
+	}
+
+	return result
+}
+
 // Listen starts the SMTPD server
 func Listen() error {
 	if config.SMTPAuthAllowInsecure {
@@ -178,6 +193,7 @@ func listenAndServe(addr string, handler smtpd.Handler, authHandler smtpd.AuthHa
 	srv := &smtpd.Server{
 		Addr:          addr,
 		Handler:       handler,
+		HandlerRcpt:   handlerRcpt,
 		Appname:       "Mailpit",
 		Hostname:      "",
 		AuthHandler:   nil,

server/ui-src/components/AboutMailpit.vue

@@ -240,19 +240,23 @@ export default {
 												</tr>
 												<tr>
 													<td>
-														SMTP messages received
+														SMTP messages accepted
 													</td>
 													<td>
-														{{ formatNumber(mailbox.appInfo.RuntimeStats.SMTPReceived) }}
-														({{ getFileSize(mailbox.appInfo.RuntimeStats.SMTPReceivedSize) }})
+														{{ formatNumber(mailbox.appInfo.RuntimeStats.SMTPAccepted) }}
+														<small class="text-secondary">
+															({{
+																getFileSize(mailbox.appInfo.RuntimeStats.SMTPAcceptedSize)
+															}})
+														</small>
 													</td>
 												</tr>
 												<tr>
 													<td>
-														SMTP errors
+														SMTP messages rejected
 													</td>
 													<td>
-														{{ formatNumber(mailbox.appInfo.RuntimeStats.SMTPErrors) }}
+														{{ formatNumber(mailbox.appInfo.RuntimeStats.SMTPRejected) }}
 													</td>
 												</tr>
 												<tr>

server/ui/api/v1/swagger.json

@@ -762,23 +762,23 @@
               "type": "integer",
               "format": "int64"
             },
-            "SMTPErrors": {
-              "description": "SMTP errors since run",
+            "SMTPAccepted": {
+              "description": "SMTP accepted messages since run",
+              "type": "integer",
+              "format": "int64"
+            },
+            "SMTPAcceptedSize": {
+              "description": "Total size in bytes of accepted messages since run",
               "type": "integer",
               "format": "int64"
             },
             "SMTPIgnored": {
-              "description": "SMTP messages ignored since run (duplicate IDs)",
+              "description": "SMTP ignored messages since run (duplicate IDs)",
               "type": "integer",
               "format": "int64"
             },
-            "SMTPReceived": {
-              "description": "SMTP messages received via since run",
-              "type": "integer",
-              "format": "int64"
-            },
-            "SMTPReceivedSize": {
-              "description": "Total size in bytes of received messages since run",
+            "SMTPRejected": {
+              "description": "SMTP rejected messages since run",
               "type": "integer",
               "format": "int64"
             },