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:

  svejk na xsara.cz
    SMTP error from remote mail server after RCPT TO:<svejk na xsara.cz>:
    host mailin1.xsara.cz [37.9.169.110]: 550 5.7.1 Service unavailable; client [62.36.20.208] blocked using dnsbl.sorbs.net