Mails getting "stuck" in Draft - why?

Over the last week or so, I've noticed emails remaining in the Drafts folder that definitely went out. I've even gotten replies from some of those mails, so I'm sure they went out. And they also appear in my Sent folder.

So why do they stay in the Drafts folder?

They were never manually saved as drafts. But Mail might have temporarily created a draft if I was in the middle of an email and walked away for a while.

But why do they stay there after the email gets sent?

Thanks,

doug