Some context: Microsoft has disabled the use of alternative email providers in Windows' built-in email app since Windows 10, and for 365 users, unless you got one of the more expensive accounts intended for large companies, then no custom domain names for your email unless you use Godaddy as registrar. They have an exclusivity deal with Microsoft. So sure, one can look at this from an authentication perspective, or s…
They are basically imposing the worst registrar ever on customers, for this particular scenario.