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:

  vlkmartin na volny.cz
    SMTP error from remote mail server after RCPT TO:<vlkmartin na volny.cz>:
    host ccx.spamfree.cz [195.24.76.171]: 554 5.7.1 <vlkmartin na volny.cz>:
    Recipient address rejected: Access denied, user does not exist