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:

  eysselt na wiky.cz
    SMTP error from remote mail server after RCPT TO:<eysselt na wiky.cz>:
    host mail.wiky.cz [188.244.57.146]: 550 5.1.1 <eysselt na wiky.cz> User unknown; rejecting