implement PLT-6469 - Send HELO request containing domain name to SMTP server (#6322)

i18n/en.json

@@ -5902,6 +5902,10 @@
   {
     "id": "utils.mail.connect_smtp.open_tls.app_error",
     "translation": "Failed to open TLS connection"
+  },
+    {
+    "id": "utils.mail.connect_smtp.helo.app_error",
+    "translation": "Failed to set HELO"
   },
   {
     "id": "utils.mail.new_client.auth.app_error",
@@ -5911,6 +5915,10 @@
     "id": "utils.mail.new_client.open.error",
     "translation": "Failed to open a connection to SMTP server %v"
   },
+  {
+    "id": "utils.mail.new_client.helo.error",
+    "translation": "Failed to to set the HELO to SMTP server %v"
+  },
   {
     "id": "utils.mail.send_mail.close.app_error",
     "translation": "Failed to close connection to SMTP server"

utils/mail.go

@@ -49,6 +49,7 @@ func newSMTPClient(conn net.Conn, config *model.Config) (*smtp.Client, *model.Ap
 		l4g.Error(T("utils.mail.new_client.open.error"), err)
 		return nil, model.NewLocAppError("SendMail", "utils.mail.connect_smtp.open_tls.app_error", nil, err.Error())
 	}
+
 	auth := smtp.PlainAuth("", config.EmailSettings.SMTPUsername, config.EmailSettings.SMTPPassword, config.EmailSettings.SMTPServer+":"+config.EmailSettings.SMTPPort)
 	if config.EmailSettings.ConnectionSecurity == model.CONN_SECURITY_TLS {
 		if err = c.Auth(auth); err != nil {
@@ -135,6 +136,15 @@ func SendMailUsingConfig(to, subject, body string, config *model.Config) *model.
 	defer c.Quit()
 	defer c.Close()
 
+	siteName := GetSiteName(*config.ServiceSettings.SiteURL)
+	if siteName != "" {
+		err := c.Hello(siteName)
+		if err != nil {
+			l4g.Error(T("utils.mail.new_client.helo.error"), err)
+			return model.NewLocAppError("SendMail", "utils.mail.connect_smtp.helo.app_error", nil, err.Error())
+		}
+	}
+
 	if err := c.Mail(fromMail.Address); err != nil {
 		return model.NewLocAppError("SendMail", "utils.mail.send_mail.from_address.app_error", nil, err.Error())
 	}

utils/utils.go

@@ -6,6 +6,7 @@ package utils
 import (
 	"net"
 	"net/http"
+	"net/url"
 	"os"
 
 	"github.com/mattermost/platform/model"
@@ -66,3 +67,11 @@ func GetIpAddress(r *http.Request) string {
 
 	return address
 }
+
+func GetSiteName(siteURL string) string {
+	u, err := url.Parse(siteURL)
+	if err != nil {
+		return ""
+	}
+	return u.Host
+}