Update mail.php (#5877)

Add `local_domain` as an option to the smtp configuration. This can be used to change the domain that is used to send the `EHLO` command during the SMTP handshake.

`null` is a sensible default since Symfony/Mailer will use it's own default (`127.0.0.1`) to send the mail.

Co-authored-by: Bram in 't Zandt <bram@bigspark.com>

config/mail.php

@@ -42,6 +42,7 @@
             'username' => env('MAIL_USERNAME'),
             'password' => env('MAIL_PASSWORD'),
             'timeout' => null,
+            'local_domain' => env('MAIL_EHLO_DOMAIN'),
         ],
 
         'ses' => [