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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...

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Is there a reason you can't use IMAP on all clients? –  grawity Jun 3 '11 at 17:17
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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.

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