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:

  zdsob na clnet.cz
    SMTP error from remote mail server after RCPT TO:<zdsob na clnet.cz>:
    host email.nej.cz [81.200.48.61]: 550 5.1.1 <zdsob na clnet.cz>:
    Recipient address rejected: User unknown in virtual mailbox table