Rainloop Mail System Error

hi all,

I have look every place, and not find solution for the follow, when send emails from rainloop:

his is the mail system at host myhostname server.

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

example@email.local: user unknown

Anyone can me help me please?

Thank you