Fix: 504 5.5.2 <localhost>: Helo command rejected

modules/mailer/mailer.go

@@ -10,6 +10,7 @@ import (
 	"net"
 	"net/mail"
 	"net/smtp"
+	"os"
 	"strings"
 
 	"github.com/gogits/gogs/modules/log"
@@ -95,6 +96,12 @@ func sendMail(settings *setting.Mailer, recipients []string, msgContent []byte)
 		return err
 	}
 
+	hostname, err := os.Hostname()
+	if err != nil {
+		return err
+	}
+	client.Hello(hostname)
+
 	// If not using SMTPS, alway use STARTTLS if available
 	hasStartTLS, _ := client.Extension("STARTTLS")
 	if !isSecureConn && hasStartTLS {