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:

  klopik na o2.cz
    SMTP error from remote mail server after RCPT TO:<klopik na o2.cz>:
    host smtp-in.o2.cz [90.176.151.98]: 550 5.1.1 <klopik na o2.cz>:
    Recipient address rejected: undeliverable address:
    host 160.218.11.33[160.218.11.33] said: 554 5.7.1 <klopik na o2.cz>:
    Recipient address rejected: User does not exist. (in reply to RCPT TO command)