Mail delivery failed: returning message to sender

[Mail Delivery System]

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:

  xyzmko na interia.cz
    SMTP error from remote mail server after RCPT TO:<xyzmko na interia.cz>:
    host interia.cz [195.88.142.164]: 550 5.7.1 Unable to relay