After mail server is changed from Courier-IMAP to Devocot (e.g. after updating Plesk), mail queue may become clogged and stop accepting incoming messages.

There will be a following error left in mail log.


Error reading configuration: Invalid settings: postmaster_address setting not given lda: Fatal: Internal error occurred. 
Refer to server log for more information. 4.2.1 Message can not be delivered at this time

You can fix this by adding postmaster_address entry under applicable domain's section in /etc/dovecot/conf.d/90-plesk-sieve.conf file:

postmaster_address = admin@domain.tld

※ please replace domain.tld with your own domain, making sure to preserve proper email address formatting.