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.

