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:

  321peto na docmail.cz
    SMTP error from remote mail server after RCPT TO:<321peto na docmail.cz>:
    host mail.dc3.cz [81.0.217.5]: 550 Recipient not found