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:

  stumbaker na sms.cz
    SMTP error from remote mail server after RCPT TO:<stumbaker na sms.cz>:
    host mxmailserver.sms.cz [89.29.94.140]: 550 User <stumbaker na sms.cz> unknown