Mail.app putting emails into Drafts folder

Since moving to a brand new iMac with Mountain Lion on it Apple Mail is putting constantly emails that I'm writing into to the Drafts folder and lets them remain there even after I have sent the email - I then have the email one time in the Drafts folder and one time in the Sent folder.

This is happening also with emails that I do not save in between. I write an email over a longer period of time, I send this email ... and I find this email later not only in the Sent folder but also in the Drafts folder.

Today it even went one step further by not allowing me to delete such a double email from the Drafts folder. Or better: putting it back into the Drafts folder again and again just seconds after I deleted it.

This is happening with emails from several IMAP accounts. This did not happen with the same IMAP accounts on my previous iMac with Lion.

Is this happening to others, too? Can someone explain what is happening here and how I can prevent this?

Thanks. I had that option active for some email accounts. It is indeed one of several very annoying bugs that Apple obviously does not care about to fix.