Dovecot cannot handle mail accounts with name containing "&". Maillog contains the following message
srv-host postfix/pipe: 9952F11F8910: to=<firstname.lastname@example.org>, relay=plesk_virtual, delay=8421, delays=8420/0.07/0/0.21, dsn=4.2.1, status=deferred (Message can not be delivered at this time )
This is a software issue with internal ID #PPPM-3065 and it will be fixed in further Plesk updates.
& (ampersand) is absent in list of allowed characters in
Create custom config file in
/etc/dovecot/conf.d/ with the following content:
auth_username_chars = abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890.-_@'&