Client certificate for relay

config.go

@@ -46,6 +46,8 @@ var (
 	command          = flagset.String("command", "", "Path to pipe command")
 	remotesStr       = flagset.String("remotes", "", "Outgoing SMTP servers")
 	strictSender     = flagset.Bool("strict_sender", false, "Use only SMTP servers with Sender matches to From")
+	remoteCert       = flagset.String("remote_certificate", "", "Client SSL certificate for remote STARTTLS/TLS")
+	remoteKey        = flagset.String("remote_key", "", "Client SSL private key for remote STARTTLS/TLS")
 
 	// additional flags
 	_           = flagset.String("config", "", "Path to config file (ini format)")
@@ -67,6 +69,36 @@ func localAuthRequired() bool {
 	return *allowedUsers != ""
 }
 
+func remoteCertAndKeyReadable() bool {
+	certSet := *remoteCert != ""
+	keySet := *remoteKey != ""
+	
+	// Both must be set or both must be unset
+	if certSet != keySet {
+		return false
+	}
+	
+	// If both are set, verify files exist and are accessible
+	if certSet && keySet {
+		if _, err := os.Stat(*remoteCert); err != nil {
+			log.Error().
+				Str("cert", *remoteCert).
+				Err(err).
+				Msg("cannot access remote client certificate file")
+			return false
+		}
+		if _, err := os.Stat(*remoteKey); err != nil {
+			log.Error().
+				Str("key", *remoteKey).
+				Err(err).
+				Msg("cannot access remote client key file")
+			return false
+		}
+	}
+	
+	return true
+}
+
 func setupAliases() {
 	if *aliasFile != "" {
 		aliases, err := AliasLoadFile(*aliasFile)
@@ -137,6 +169,11 @@ func setupRemotes() {
 				logger.Fatal().Msg(fmt.Sprintf("error parsing url: '%s': %v", remoteURL, err))
 			}
 
+			if *remoteCert != "" && *remoteKey != "" && (r.Scheme == "smtps" || r.Scheme == "starttls") {
+				r.ClientCertPath = *remoteCert
+				r.ClientKeyPath = *remoteKey
+			}
+
 			remotes = append(remotes, r)
 		}
 	}
@@ -253,6 +290,13 @@ func ConfigLoad() {
 		log.Warn().Msg("no remotes or command set; mail will not be forwarded!")
 	}
 
+	if !remoteCertAndKeyReadable() {
+		log.Fatal().
+			Str("remote_certificate", *remoteCert).
+			Str("remote_key", *remoteKey).
+			Msg("remote_certificate and remote_key must both be set or both be empty")
+	}
+
 	setupAllowedNetworks()
 	setupAllowedPatterns()
 	setupAliases()

remotes.go

@@ -14,6 +14,8 @@ type Remote struct {
 	Port            string
 	Addr            string
 	Sender          string
+	ClientCertPath  string
+	ClientKeyPath   string
 }
 
 // ParseRemote creates a remote from a given url in the following format:

smtp.go

@@ -340,6 +340,14 @@ func SendMail(r *Remote, from string, to []string, msg []byte) error {
 			ServerName:         r.Hostname,
 			InsecureSkipVerify: r.SkipVerify,
 		}
+		// Load client certificate on-demand, just before connection
+		if r.ClientCertPath != "" && r.ClientKeyPath != "" {
+			cert, err := tls.LoadX509KeyPair(r.ClientCertPath, r.ClientKeyPath)
+			if err != nil {
+				return err
+			}
+			config.Certificates = []tls.Certificate{cert}
+		}
 		conn, err := tls.Dial("tcp", r.Addr, config)
 		if err != nil {
 			return err
@@ -366,6 +374,14 @@ func SendMail(r *Remote, from string, to []string, msg []byte) error {
 				ServerName:         c.serverName,
 				InsecureSkipVerify: r.SkipVerify,
 			}
+			// Load client certificate on-demand, just before use
+			if r.ClientCertPath != "" && r.ClientKeyPath != "" {
+				cert, err := tls.LoadX509KeyPair(r.ClientCertPath, r.ClientKeyPath)
+				if err != nil {
+					return err
+				}
+				config.Certificates = []tls.Certificate{cert}
+			}
 			if testHookStartTLS != nil {
 				testHookStartTLS(config)
 			}

smtprelay.ini

@@ -119,6 +119,10 @@
 ; Mailjet.com
 ;remotes = starttls://user:pass@in-v3.mailjet.com:587
 
+; Exchange Online (O365) SMTP relay
+; (Change netloc to your own Exchange MX endpoint)
+; remotes = starttls://contoso-com.mail.protection.outlook.com:25
+
 ; Ignore remote host certificates
 ;remotes = starttls://user:pass@server:587?skipVerify
 
@@ -131,5 +135,11 @@
 ; Multiple remotes, space delimited
 ;remotes = smtp://127.0.0.1:1025 starttls://user:pass@smtp.mailgun.org:587
 
+; Client SSL certificate for remote STARTTLS/TLS
+; remote_certificate = /path/to/certificate-chain.pem
+
+; Client SSL private key for remote STARTTLS/TLS
+; remote_key = /path/to/private-key.pem
+
 ; Pipe messages to external command
 ;command = /usr/local/bin/script