If I send an html message it will come back with this error.
If, on the other hand, I send it as a plain text message, the recipient will receive it.
What could be the problem?

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:

****wap@freemail.hu
host fmx.freemail.hu [84.2.43.65]
SMTP error from remote mail server after end of data:
550 5.7.1 Message denied by policy