We are running Odoo 13 online, after an internal business change which meant a change to our domain, our email stopped working. Now we get the error;

Mail Delivery Failed Mail delivery failed via SMTP server 'None'. SMTPDataError: 554 5.2.0 STOREDRV.Submission.Exception:SendAsDeniedException.MapiExceptionSendAsDenied

I tried configuring the external SMTP sender etc  only to find that it doesnt work well with out o365 setup. I have unticked it but if I need to delete the SMTP box, then I can. 

I have changed the catchall, alias and domain to our new domain... Maybe that is why?

What is the default setting for Odoo online to get the built-in mail server to work and how do I revert back to it but with our new domain. Does Odoo have to update something for us?