Sent messages disappear and don't arrive. (Receiving is ok)

Hi List,

I'm running an email server (Postfix, Dovecot, MySQL) on an RPi. I set it up using this 'howto' https://pestmeester.nl/index.html#11.0. It seemed to work but now sent emails just disappear and are not received. Inward emails are ok.

I'm a Linux and Postfix Dovecot learner so please treat me as not being very knowledgeable. I'm not even sure if this is a Postfix or Dovecot issue so I've posted to both. (Nothing from Postfix so far!)

I've tried to include what relevant info I thought a helper might need but if I've missed something please let me know.

I'll appreciate any help.

TIA,

John