see if this makes Expunge any clearer (I *believe* gmail defaults to auto-expunge on the server side)
?The IMAP protocol allows messages to be marked for deletion, a sort of limbo state where a message is still present in the folder but is slated to be deleted the next time the folder is expunged or compacted. When we launched Gmail IMAP, we thought it would confuse people to have messages in this state, since there?s nothing like it in the web interface (the Trash label works differently). So in our IMAP implementation, when you mark a message as deleted, Gmail doesn?t let it linger in that state ? it goes ahead and deletes it from the folder completely. We call this auto-expunge. This may have saved some people from confusion, but other users still wanted the two-stage delete process.?
---Gmail engineer Jamie Nicolson.