Thank you UFHH01 for your advice.
According to the Plesk support people, it turns out to be this error, as yet unresolved for me:
Unable to receive e-mails with Dovecot: Error: userdb lookup: connect(/var/run/dovecot/auth-userdb) failed: Permission denied
Hello all
I've configured a mail account on a new instance of Plesk Onyx. Outgoing mail is fine, using webmail or a mail client. But incoming mail is permanently queued, no matter who the sender is.
If you have any suggestions I would love to hear them.
Many thanks :)
From /var/logmaillog...