config(mail): fix SERVER_NAME; closes #4600

.env.example

@@ -41,4 +41,4 @@ MAIL_FROM_NAME="Pterodactyl Panel"
 # mail servers such as Gmail to reject your mail.
 #
 # @see: https://github.com/pterodactyl/panel/pull/3110
-# SERVER_NAME=panel.example.com
+# MAIL_EHLO_DOMAIN=panel.example.com

config/mail.php

@@ -41,7 +41,7 @@ return [
             'username' => env('MAIL_USERNAME'),
             'password' => env('MAIL_PASSWORD'),
             'timeout' => null,
-            'local_domain' => env('MAIL_EHLO_DOMAIN'),
+            'local_domain' => env('MAIL_EHLO_DOMAIN', env('SERVER_NAME')),
         ],
 
         'ses' => [