Receiving "553 Relaying Disallowed " after problems with nameservers

Receiving "553 Relaying Disallowed " after problems with nameservers

Hi,

Please could you help?
I've been using Zoho for years and love it. I setup my domain hosted e-mail through Zoho for the domain tec-tweak.net and everything's been working fine for years.
I recently had an issue with the nameservers for my domain which has been fixed.
Since then I am now receiving "553 Relaying disallowed" when e-mailing my mail account nicmagic@tec-tweak.net

I've confirmed the address is correct, confirmed my MX records are correct (Although I even changed them to euro servers as suggested by the help pages but just recently put them back to the .com servers)
My domain is verified and set as primary.

Please help. I appreciate your time.
Thank you
Nic