Duplicated sent messages using Apple Mail.app

Hello,

I have set up an IMAP Outlook account with a custom domain on Apple Mail.app. Everything works well so far, but when I send an email, two identical copies of the same email are stored in the Sent folder (but one copy is sent to the recipient). In Mail.app I have tried everything.

Since 2 month the same problem and nobody is helping. They always keep saying, write in another area or open a own thread.

So please held me solving this problem - finally.

Thanks!