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:

  jarda na mboxzn.cz
    SMTP error from remote mail server after RCPT TO:<jarda na mboxzn.cz>:
    host ms.oknet.cz [193.165.254.8]: 550 5.1.1 <jarda na mboxzn.cz>... User unknown