set EHLO domain in SMTP check

Application/Controller/Admin/SmtpChecker.php

@@ -124,7 +124,7 @@ class SmtpChecker extends AdminDetailsController
     {
         $this->action = __FUNCTION__;
         Assert::that(
-            ($this->smtp = new Net_SMTP($this->host, $this->port)),
+            ($this->smtp = new Net_SMTP($this->host, $this->port, $_SERVER['HTTP_HOST'])),
             Registry::getLang()->translateString('D3_MAILCHECKER_SMTPCHECK_INSTANCE')
         )->isInstanceOf(
             Net_SMTP::class,