I have always used Citadel as my email server on the Raspberry Pi, and one of the great aspects is that it has just integrated with WordPress and its PHP mail function. (For example, the Sucurri Security function to email me alerts and stats works with no intervention needed on my part.).
Unfortunately there is an issue that prevents Citadel from working on the Debian Buster destro. The issue appears to be related to the Buster build, but I cannot confirm this, nor do I know whether it is a Debian Buster build issue or Citadel issue, or a combination of both. However, as of October 2019, Citadel does not work on Debian Buster installations, and, as I installed Buster on my Pi 4, I am unable to use Citadel on this server.
The simplest solution for me was to keep Citadel running on the Pi 3 as my email server, whilst using the Pi 4 as my web server. This solution ensured I retained all the quintic.co.uk mail accounts, including, crucially firstname.lastname@example.org.
However, this left the Pi4 without a MTA (Message Transfer Agent), and so the PHP mail function no longer functioned . i.e. No e-mails from WordPress.
I therefore needed to set-up a MTA on my Pi4. There are two pure MTAs that appear to have traction for the Raspberry Pi: Exim and SSMTP. I do not know which is considered the better MTA, but at the time of writing there was no package set for Exim on Debian Buster for the Pi4. So, for me, the choice was easy – SSMTP.
apt-get install ssmtp mailutils mpack cd /etc/ssmtp nano ssmtp.conf - include the following lines. mailhub=pi3 - My server running citadel AuthUser=<user for Citadel> AuthPass=xxxxxxxx useSTARTTLS=YES cd /etc/php/7.x/apache2 nano php.ini sendmailpath=/usr/sbin/ssmtp -t -- Look for the line ;sendmail and replace.
The above worked nicely for me.