Broken SMTP after switching to PHP 7.2
The old warning was actually just dealt with yesterday (https://github.com/civicrm/civicrm-core/pull/13770).

If you can remove mcrypt, you can get smtp working again by going to the admin screens in civicrm to reenter and resave your password. My understanding is it will be stored unencrypted in the database, but at least will work.