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:

  dzierza na seznam.cz
    SMTP error from remote mail server after RCPT TO:<dzierza na seznam.cz>:
    host mx1.seznam.cz [77.75.76.42]: 550 5.1.1 Sorry, no mailbox here by that name.
  proctone na seznam.cz
    SMTP error from remote mail server after RCPT TO:<proctone na seznam.cz>:
    host mx1.seznam.cz [77.75.76.42]: 550 5.1.1 Sorry, no mailbox here by that name.
  3hm.1hb6v.5x2gny32mhz.1mbqrq na seznam.cz
    SMTP error from remote mail server after RCPT TO:<3hm.1hb6v.5x2gny32mhz.1mbqrq na seznam.cz>:
    host mx1.seznam.cz [77.75.76.42]: 550 5.1.1 Sorry, no mailbox here by that name.
  petra.m18 na seznam.cz
    SMTP error from remote mail server after RCPT TO:<petra.m18 na seznam.cz>:
    host mx1.seznam.cz [77.75.76.42]: 550 5.1.1 Sorry, no mailbox here by that name.