@scottalanmiller Ok, I think I got it. Per http://bugs.elastix.org/view.php?id=2549:

Starting with PHPMailer v5.2.7, the default HELO hostname is not set anymore to 'localhost' if not supplied on the Hello() function... This generates that the Elastix remotesmtp configuration checker ends up sending an empty HELO/EHLO command, which makes Gmail (and probably many other SMTP servers) really unhappy.

A possible solution would be to modify /var/www/html/modules/remote_smtp/libs/paloSantoEmailRelay.class.php on line 156:

if(!$smtp->Hello()){

change to:

if(!$smtp->Hello(getHostname())){

If you are using the Multi Tenant version the location is /usr/share/elastix/apps/remote_smtp/libs/paloSantoEmailRelay.class.php

Changed that and it worked. However, whenever I create a new organization and the email is sent to the "admin", I never receive anything... what could it be now? Any ideas? Thanks!