So IMAP has a feature where, once I've looked at a mailbox, I can efficiently look for any new messages by asking it for any new UIDs which I haven't seen yet.
But I how do I efficiently find expunged messages? I haven't found any commands for doing this yet; my only option is to retrieve the full UID list of the mailbox and looking for missing ones. This is rather suboptimal.
I have mailboxes with 25000+ messages. Scanning one of these mailboxes takes megabytes of traffic just to do the UID SEARCH command, so I'd like to avoid this. Is there anything hidden in the depths of the IMAP protocol that I'm missing?
Okay, for offline use this can work:
Since big mailboxes typically grow big by having many messages added and few deleted, you can optimise for there being large unchanged parts of the messages. If your clientside store contains 10000 messages, then you can send 'x UID SEARCH 2000,4000,6000,8000,10000` which will return five UIDs in one SEARCH response. Depending on which ones have changed, you know whether there have been any expunges in each fifth of the mailbox, so if the mailbox hasn't changed you've verified that very cheaply. If a particular fifth has changed, you can retrieve all 2000 UIDs in that.
QRESYNC/CONDSTORE is much nicer though, and also let you resync flag state.