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 = firstname.lastname@example.org
※ please replace domain.tld with your own domain, making sure to preserve proper email address formatting.