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:

  knazek.lukas na homecity.cz
    SMTP error from remote mail server after RCPT TO:<knazek.lukas na homecity.cz>:
    host mail.poda.cz [185.119.216.223]: 550 5.1.1 <knazek.lukas na homecity.cz> User unknown; rejecting