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:

  karel.cernik na matfyz.cz
    SMTP error from remote mail server after end of data:
    host pluto.ms.mff.cuni.cz [195.113.20.27]: 554 5.7.1 Spam message rejected