Draft emails not deleting after sending via Mail.app

When I compose an email a copy of that get auto saved in the Draft folder and should delete itself once I press send but that is not the case. After I press send I can see a copy in my Sent folder AND in the Draft folder. It only seems to happen with some emails that have attachments even with very small file sizes.

Has anyone else had this problem and is there a fix?

Email is my business and I can't have unpredictable behaviour like this. When I see the same message in my Draft and Sent folders its impossible to tell if it has actually been sent. I have to follow up with a phone call to verify receipt. Ridiculous!

Here are my relevant specs:

• IMAP email service
• OSX 10.6.8
• Mail.app

If you have a fix, thanks in advance.