I too have been struggling with the problem of Sent mail not appearing in the Sent folder, using Mail. I've also had intermittent problems when trying to Archive messages and moving emails from one folder to another.
As previously posted, it appears to be a problem / conflict between certain ISPs security protocols and Apple Mail having non-standard commands.
This doesn't seem to effect everybody, but it effects me, and has resulted in me "loosing" several sent messages (although the recipient has recieved them, I have no record of them, not even on the webmail interface). I've also had problems with attachments from incoming mail not being visible or just being corrupted (although I am not sure if this is related).
I've noticed the sent mail problem is made marginally better if:
- The mail message is of small size (ie there is no attachment)
- You switch off "store Drafts Server side" (ie store them locally) as this is constantly trying to save your message to an IMAP server and just makes the problem worse
- Using the Addon "MailHub" can improve things
Despite all of this i'm still occasionally loosing sent mails and having problems moving / archiving / deleting emails. The Apple Mail - IMAP interface just isn't reliable for me.
My email provider is via Yahoo, and I've noticed that many people who have this problem are using Yahoo and Apple Mail. In Yahoo's defence, they never claim to support IMAP for desktop systems, only portable IOS devices, so that leaves me with a choice of either changing my email provider or my email client.
Until this problem is resolved, I've opted to change my email client and am trying out "Postbox". It basically Thunderbird underneath but with an interface which is much more apple-mail like. Unfortunately this solution isn't free, but it will do for the moment. Postbox can be do alot of things apple mail can (after a bit of tweeking in the settings), has smart folders and a decent search. So far I haven't had a single hiccup, and it has proved 100% reliable, so will stick with it until this problem is resolved.