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:

  sgfs na wt.cz
    SMTP error from remote mail server after RCPT TO:<sgfs na wt.cz>:
    host wt.cz [81.19.186.40]: 550 No Such User Here