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:

  dvorakjan na sssvt.cz
    SMTP error from remote mail server after RCPT TO:<dvorakjan na sssvt.cz>:
    host mail.sssvt.cz [91.139.15.99]: 550 5.1.1 Mailbox <dvorakjan na sssvt.cz> does not exist