Drafts is not just for autosaving of emails.
 

The Drafts folder is (and has always been) where Mail.app saves partially completed emails. When you go to close the window of an email that you have started to compose, it goes in Drafts - which is the logical place for it.

But the OP mentioned (and I have this issue to this day) that these messages are in the Drafts boxe even after they are sent.

I was only responding to szbotnik's statement that " 'drafts' folder is ONLY for AUTOSAVE functions". It is not.

Also note that my system still suffers from the bug.

I just switched my MacMail from POP to IMAP and was having the same issues.
I followed the Apple chat transcript from big evil's post on 12-01-2010 and it worked like a charm.

I've had this same issue for some time, in 10.7.3 now too. It seems more likely to happen if the email has an attachment, and/or if I put my own reply-to address in.

Sometimes (as this morning) I come back to find *two* copies of the same message in my drafts folder.

Overzealous.

I followed bigevil's transcript too and it has worked for me.

Let's hope this fix is persistent.