Dimos Alevizos <dalevizo@otenet.gr> wrote:

We recently completed the migration in our company from qpopper/mbox to dovecot/mbox using only pop3_reuse_xuidl = yes and while everything went much easier than we feared we came across an interesting problem that you might want to pay attention to.

We use dovecot to provide both pop3 and imap (directly and via webmail) access to our client's mailboxes and we noticed that when a client moves a mail to a different folder and then back into INBOX (say he deleted it by mistake and moves it back), dovecot inserts the same X-UIDL into it's cache, thus creating duplicate entries.

When a pop3 client sees this (at least outlook in our case) it think's that there's something wrong with the server's X-UIDL handling and re-downloads the duplicate messages. EVERY time the user checks his mails.

I haven't heard any user complaint about this, but perhaps nobody tried this before.

I recreated this situation (copy old qpopper message INBOX -> tmpbox -> INBOX) and produced two messages with identical X-UIDL headers, but according to the Wikipedia entry for POP3:

Comparison with IMAP

Clients that leave mail on servers generally use the UIDL command
to get the current association of message-numbers to message
identified by its unique identifier. The unique identifier
is arbitrary, and might be repeated if the mailbox contains
identical messages.

This is echoed in RFC1939 (page 11-12)

While it is generally preferable for server implementations
to store arbitrarily assigned unique-ids in the maildrop,
this specification is intended to permit unique-ids to be
calculated as a hash of the message.  Clients should be able
to handle a situation where two identical copies of a
message in a maildrop have the same unique-id.

So POP3 UIDLs are not guaranteed to be unique, especially if the messages are the same. Perhaps some mail readers are making unwarranted assumptions about the uniqueness of UIDLs:

http://www.eggheadcafe.com/software/aspnet/33485042/duplicate-messageiduidlleave-on-serverrepeated-downloads-of-em.aspx
https://www.ritlabs.com/bt/view.php?id=3599

Joseph Tam <jtam.home@gmail.com>