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.<p>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.