[Dovecot] Feature wish: "Virtual" POP3 folder with IMAP

[jbdovecot at fcgh.net]

Hallo,

is it possible to implement a "flat folder view" for POP3 users?  

The problem is that POP3 only knows one folder: INBOX.  Now, if a user logs
in via POP3, he won't see messages that live outside the INBOX folder - this
could be e.g. a *SPAM*-flagged message moved to another folder via
Sieve/LDA/other mechanism.

What about adding a feature `pop3_remap_folders` to virtually place *all*
messages spread across different folders inside INBOX, only for the POP3
session, and without impact on the actual folder structure as seen thru
IMAP?

I see some issues though:

	- How would SENT and TRASH be handled?  These special folders
shouldn't be part of the game.

	- What happens if the POP3 client deletes a message, which is the
default behavior of Outlook and friends?  Proposed logic: really delete
message if not logged in via IMAP since ($filemtime_of_message -
$user_configurable_amount_of_seconds), else gracefully ignore client's
delete request.  This *could* become a problem if quotas are enabled, but
then again it should be the admin's decision to offer this feature to his
users.

	- Message IDs: Would the above Delete-only-if workaround confuse
some mail clients and make them download the same messages over and over?

	- Perhaps another way: create a separate POP3 directory inside the
mailbox, only used by dovecot-pop3.  Inside reside symlinks to the actual
messages spread across different IMAP folders, which can safely be deleted
by a POP3 client and also following the real-delete logic described above.

The benefit of such a virtual folder remapping would be great for systems
also using webmail and/or Sieve.

Regards,
James

--
jbdovecot at fcgh.net