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

[John]

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
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