Check the postfix queue, I ran sendmail servers for a number of years. I don’t know anything about postfix but I would try that if your mails cannot be sent.

On Sun, Jul 14, 2019 at 2:21 PM @lbutlr via dovecot <dovecot@dovecot.org> wrote:

On 14 Jul 2019, at 03:35, John via dovecot <dovecot@dovecot.org> wrote:
> 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.

You will need to look in your postfix logs to see what is happening to outbound mails.

If you are on a home connection, most ISPs do not allow mail servers.

--
Science is the foot that kicks magic square in the nuts.