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:

  vaclav.krauz na cdcargo.cz
    SMTP error from remote mail server after end of data:
    host ax.virusfree.cz [31.7.241.24]: 554 message refused (#5.7.1) - for more information visit https://www.virusfree.cz/cs/help