I have been running a number of Wordpress sites based on a RPi 3 with SSD attached based on a previous version of Pestmeester's excellent guide (https://pestmeester.nl/index.html#10.0). I installed only the web server side, not the email solution. It has proven to be a secure, reliable and inexpensive solution.
However, sending emails from the Wordpress app has never succeeded (e.g. iThemes backups, etc.)
Any ideas or guides on how to enable this?