Add support SMTP HELO

README.md

@@ -46,13 +46,22 @@ On local PC (192.168.1.54) create file `/etc/mailrelay.json` with contents:
     "smtp_password": "secretAppPassword",
     "smtp_max_email_size": 10485760,
     "smtp_login_auth_type": false,
+    "smtp_helo": "smtp.example.com",
     "local_listen_ip": "0.0.0.0",
     "local_listen_port": 2525,
     "allowed_hosts": ["*"],
     "timeout_secs": 30
 }
 ```
 
+Notes on the configuration:
+
+- `smtp_helo` is optional - when given the client will present itself
+  to the upstream with the given name.
+- `smtp_username` is optional - if given the client will attempt to
+  authenticate itself with the server.  You should also give a password
+  in this case.
+
 Run `mailrelay`,
 
 ```Bash

client.go

@@ -92,21 +92,30 @@ func sendMail(e *mail.Envelope, config *relayConfig) error {
 }
 
 func handshake(client *smtp.Client, config *relayConfig, tlsConfig *tls.Config) error {
+	if config.HeloHost != "" {
+		if err := client.Hello(config.HeloHost); err != nil {
+			return errors.Wrap(err, "HELO error")
+		}
+	}
+
 	if config.STARTTLS {
 		if err := client.StartTLS(tlsConfig); err != nil {
 			return errors.Wrap(err, "starttls error")
 		}
 	}
 
-	var auth smtp.Auth
+	var auth smtp.Auth = nil
+
 	if config.LoginAuthType {
 		auth = LoginAuth(config.Username, config.Password)
-	} else {
+	} else if config.Username != "" {
 		auth = smtp.PlainAuth("", config.Username, config.Password, config.Server)
 	}
 
-	if err := client.Auth(auth); err != nil {
-		return errors.Wrap(err, "auth error")
+	if auth != nil {
+		if err := client.Auth(auth); err != nil {
+			return errors.Wrap(err, "auth error")
+		}
 	}
 	return nil
 }

main.go

@@ -30,6 +30,7 @@ type mailRelayConfig struct {
 	SMTPLoginAuthType bool     `json:"smtp_login_auth_type"`
 	SMTPUsername      string   `json:"smtp_username"`
 	SMTPPassword      string   `json:"smtp_password"`
+	SMTPHelo          string   `json:"smtp_helo"`
 	SkipCertVerify    bool     `json:"smtp_skip_cert_verify"`
 	MaxEmailSize      int64    `json:"smtp_max_email_size"`
 	LocalListenIP     string   `json:"local_listen_ip"`

server.go

@@ -47,6 +47,7 @@ func Start(appConfig *mailRelayConfig, verbose bool) (err error) {
 		"smtp_starttls":         appConfig.SMTPStartTLS,
 		"smtp_login_auth_type":  appConfig.SMTPLoginAuthType,
 		"smtp_skip_cert_verify": appConfig.SkipCertVerify,
+		"smtp_helo":             appConfig.SMTPHelo,
 	}
 	cfg.BackendConfig = bcfg
 
@@ -64,6 +65,7 @@ type relayConfig struct {
 	Username      string `json:"smtp_username"`
 	Password      string `json:"smtp_password"`
 	SkipVerify    bool   `json:"smtp_skip_cert_verify"`
+	HeloHost      string `json:"smtp_helo"`
 }
 
 // mailRelayProcessor decorator relays emails to another SMTP server.