From time to time my Drafts folder contains an autosaved message that I haven't sent which is nice because I can pick up where I left off creating a new message, but lately most of the messages are messages that I did send.
I then have to go and check that the message did in-fact get sent before I can delete the draft.
This mailbox is unified from 3 IMAP accounts, one being a gmail account. I haven't seen a pattern (e.g. the zombie drafts always being from one account).
Has anyone else fixed this issue, or at least seen it?