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:

  grygar na vlrz.cz
    SMTP error from remote mail server after RCPT TO:<grygar na vlrz.cz>:
    host vlrz-cz.mail.protection.outlook.com [104.47.2.36]:
    550 5.4.1 Recipient address rejected: Access denied. AS(201806281) [DB5EUR01FT018.eop-EUR01.prod.protection.outlook.com]