Draft emails aren't automatically deleted when sent
I'm using Mail 8.2 on a MacBook Pro with OS X Yosemite. Draft emails are stored in my Drafts folder, either automatically or if I save them. On earlier systems, they would automatically be deleted when the email was sent. Now they remain, so I can be unsure later whether or not I have actually sent them. How do I stop this happening?
MacBook Pro, Mac OS X (10.7.5), null