Recently I have been on a slow internet connection, and have had trouble checking my mail with Mail.app. The previous day, I had sent an email with large attachments, and every time Mail checked for new messages, it wanted to upload that email to my Drafts folder on my IMAP account.
Deleting the draft didn't help, nor did changing settings to not store drafts on the server. Eventually I figured out there is a hidden folder containing these pending operations. To cancel any pending operations for an IMAP account, delete all the files in the following folder: ~/Library » Mail » IMAP-youremail@somedomain.com » .OfflineCache
Why Mail wants to upload a draft of an email I had already sent is another issue, and I believe it has to do with my slow internet connection: As I'm writing the email, it periodically tries to save a draft, but it fails. When it fails, it remembers to try again, but doesn't figure out that it's no longer necessary.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080906033905844