Postfix user unknown problem

Hey i get since some days this problem:

"This is the mail system at host xxxxx.xxxxxx-xxxxxx.xx.

I’m sorry to have to inform you that your message could not
be delivered to one or more recipients. It’s attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

               The mail system user unknown"

The confusing thing is this happens only on 1 email adress the rest on the same domain is working fine.

Hi Pimmal, Can you fix the problem?