[Dovecot] "leave mail on server" workaround

Hello,

POP users often keep fetched mail on server, by checking for "leave mail on server" or similar option in their client.

What would be the best way to disallow them using such approach. That is, what is the best way to force deletion of messages that have been RETRieved in a POP session at the end of that POP session?

RETRieved messages will be marked as \Seen, but since users might also use an IMAP client (e.g. webmail), it'll be dangerous to simply delete all \Seen messages in user's mailbox.

I'm all open for suggestions.

-- -- Kirill Miazine km@krot.org