[matt_heimer]

Hadn't heard about the custom domains on 365, upon researching there is a limitation - "At the moment, we only support connecting domains managed by GoDaddy with Outlook.com" - https://support.microsoft.com/en-us/office/get-a-personalize...

[iofiiiiiiiii]

That limitation seems to just be a marketing thing - a few more searches reveal they do not actually check what registrar you use.