I have only experience with UW-IMAP. It works very well for me, on a PPro 233 MHz. It uses ~/.mailboxlist with Mozilla mail, which is a convenient way to store subscribed folders in.

Mail.app us using the wrong folder for "Sent" for a particular imap server. I remember, and an internet search confirms, that Mail->Preferences->Accounts used to have entries for this. The versions I ...