solve admin problems by recommending it in setup, not just FAQ.

setup/templates/steps/compose/03_expose.html

@@ -50,7 +50,9 @@ avoid generic all-interfaces addresses like <code>0.0.0.0</code> or <code>::</co
 
 <p>You server will be available under a main hostname but may expose multiple public
 hostnames. Every e-mail domain that points to this server must have one of the
-hostnames in its <code>MX</code> record. Hostnames must be coma-separated.</p>
+hostnames in its <code>MX</code> record. Hostnames must be coma-separated. If you're having
+trouble accessing your admin interface, make sure it is the first entry here (and possibly the
+same as your <code>DOMAIN</code> entry from earlier.</p>
 
 <div class="form-group">
   <label>Public hostnames</label>