Error clearly indicates AAD account created by someone else on their service. This is not "M$FT" coming after a small fish. That's just their corp system preventing corp customers to create multiple accounts

Noticed how much spam comes from Microsoft Outlook users. A consistent deluge of spammy service offers. If a smaller provider allowed that, their emails would be blacklisted.

Microsoft not allowing Tutanota users for team while allowing Gmail (Google Mail) is just more proof of the email cartel. Major email providers help major email providers.