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:

  fuchs23 na sezna.cz
    SMTP error from remote mail server after RCPT TO:<fuchs23 na sezna.cz>:
    host sezna.cz [46.28.105.4]: 554 5.7.1 Service unavailable; Client host [62.36.20.208] blocked using dnsbl.sorbs.net; Currently Sending Spam See:
    http://www.sorbs.net/lookup.shtml?62.36.20.208