Unable to send emails from a free Zoho account but can receive emails.

Error is 'Unable to send message;Reason:553 Relaying disallowed. Invalid Domain'.

Saying mx records are incorrect, but they definitely are correct for Zoho requirements on VentraIP control panel.

I am in Australia and the domain is .com.au but when I change the Zoho mx records to .com.au I can no longer receive emails either, so I believe it should be .com.

Any help? thanks