Another receiving mail issue

Another receiving mail issue

Hi Zoho Team,

I am using a free account and verified my custom domain.
I also added a couple of users that can access the account without any problems.

Now, when I am sending an email to any zoho-email-accounts I am constantly receiving the following error:
  1. The error that the other server returned was: 553 553 sorry, that domain isn't allowed to be relayed thru this MTA (#5.7.1) (state 13).
I set up the MX records over an hour ago and I know I should wait for the results but using a online MX checker tools I already see the MX records are pointing correctly to the zoho servers.

Could you please give me a hint about how to solve my problem?

Thanks for any feedback and best regards,
Pawel