IMAP/POP3 weirdness: "deleting" email using iPhone causes POP3 UIDL to change?

Question

When accessing e-mail by POP3, many e-mail clients identify messages they have already downloaded using an unique ID assigned by the server (shown by the POP3 UIDL command - I'll call it UID). On my mail server, moving an e-mail into a different folder using the web interface keeps the UID for an e-mail the same, as expected.

If the e-mail account is accessed by IMAP using an iPhone (3GS I think), and the user "deletes" an e-mail, it gets moved to a different non-special folder "Deleted Messages" on the server. The problem is, this changes the UID of the e-mail, so any POP3 client will download the message again.

Has anyone ever seen this? Is there a way to avoid the UID from changing when moving the message to a "Deleted Messages" folder?

Update:

I've contacted the service desk of the e-mail provider; it is a rather anonymous hotline, but at least there is one. They claim that the iPhone does a message move to another folder by a message copy, followed by deleting the old message. As mentioned above, the same effect does not appear when using their web interface directly. They seemed to claim this to be a limitation of IMAP. Is this true, does IMAP not allow a "move" operation between folders? I've read that message IDs in IMAP are folder dependent, but surely this does not have to mean that the POP3 UIDs have to change as well...

Answer 1

Is this true, does IMAP not allow a "move" operation between folders?

It's true; IMAP (as of v4rev1) does not include a MOVE command.

but surely this does not have to mean that the POP3 UIDs have to change as well...

When you do a "move" in IMAP, by copying the message and deleting the original, you are essentially creating a new message with the same contents. And since it is a new message, it gets a new UID in both IMAP and POP3.