My Zoho email addresses that are linked to a domain suddenly stopped receiving messages yesterday, even though I've been using Zoho mail without problems for 1,5 years. I receive this reply to test emails: "553 Relaying disallowed". Due to this, my SMTP plugin doesn't work either  ("SMTP error: could not authenticate"). Domain's MX Records are pointed to Zoho correctly.

So what is the problem here? I haven't changed any settings before the problem occured, also Wordpress plugins are updated.

Thanks for you help!