Mail delivery error


I have a problem, because I receive mail that my review request was not delivered. I have tried to send review request to different (working) e-mails and it has failed.

I recieved e-mail in return, please see bellow.

Can you give me any suggestion what could be wrong?

e-mail message:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

host []
SMTP error from remote mail server after RCPT
550-5.1.1 Recipient address rejected:
User unknown in relay recipient table
550 5.1.1 For assistance, see or contact +358404844333. Please provide the following information in your problem report:
Time: (May 15 14:41:24), Client: (, Server: