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:

  eva na sixta.cz
    SMTP error from remote mail server after end of data:
    host mx1.mailchannels.net [44.236.199.4]: 550 5.7.1 Message rejected