Feature: Workaround for non-RFC-compliant message headers containing …

…<CR><CR><LF>

Due to a bug in some common sendmail implementations and PHP >=8.0, message headers sometimes contain `\r\r\n` which is not RFC compliant.

Mailpit will now fix these non-compliant headers. This can be disabled via `--smtp-strict-rfc-headers`

See #87 / #153

cmd/root.go

@@ -100,6 +100,7 @@ func init() {
 	rootCmd.Flags().StringVar(&config.SMTPTLSCert, "smtp-tls-cert", config.SMTPTLSCert, "TLS certificate for SMTP (STARTTLS) - requires smtp-tls-key")
 	rootCmd.Flags().StringVar(&config.SMTPTLSKey, "smtp-tls-key", config.SMTPTLSKey, "TLS key for SMTP (STARTTLS) - requires smtp-tls-cert")
 	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().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!)")
@@ -185,6 +186,9 @@ func initConfigFromEnv() {
 	if getEnabledFromEnv("MP_SMTP_AUTH_ALLOW_INSECURE") {
 		config.SMTPAuthAllowInsecure = true
 	}
+	if getEnabledFromEnv("MP_STRICT_RFC_HEADERS") {
+		config.SMTPStrictRFCHeaders = true
+	}
 
 	// Relay server config
 	if len(os.Getenv("MP_SMTP_RELAY_CONFIG")) > 0 {

config/config.go

@@ -90,6 +90,10 @@ var (
 	// SMTPRelayConfig to parse a yaml file and store config of relay SMTP server
 	SMTPRelayConfig smtpRelayConfigStruct
 
+	// SMTPStrictRFCHeaders will return an error if the email headers contain <CR><CR><LF> (\r\r\n)
+	// @see https://github.com/axllent/mailpit/issues/87 & https://github.com/axllent/mailpit/issues/153
+	SMTPStrictRFCHeaders bool
+
 	// ReleaseEnabled is whether message releases are enabled, requires a valid SMTPRelayConfigFile
 	ReleaseEnabled = false
 

server/smtpd/smtpd.go

@@ -17,6 +17,12 @@ import (
 )
 
 func mailHandler(origin net.Addr, from string, to []string, data []byte) error {
+	if !config.SMTPStrictRFCHeaders {
+		// replace all <CR><CR><LF> (\r\r\n) with <CR><LF> (\r\n)
+		// @see https://github.com/axllent/mailpit/issues/87 & https://github.com/axllent/mailpit/issues/153
+		data = bytes.ReplaceAll(data, []byte("\r\r\n"), []byte("\r\n"))
+	}
+
 	msg, err := mail.ReadMessage(bytes.NewReader(data))
 	if err != nil {
 		logger.Log().Errorf("[smtpd] error parsing message: %s", err.Error())