Processes in Microsoft 365 for setting up Office apps, redeeming product keys, and activating licenses.
I had this happen last week. Signed into one of the live accounts I use to register for a client to activate another license for a new PC and got the "We've detected something unusual about this sign-in." message, along with the suggestion that this account was spewing forth spam. Unlikely, since the only thing this account exists for is activating Office licenses.
I think that in addition to their attempts to impose this absurd paradigm, they've also got a royally screwed up back end with this thing that is only going to get worse. They know it, and that's probably why nobody is saying anything about it because it's now too big of a knot to untie.
The same thing happened to me. I purchased a domain explicitly to be used for Office registrations. Each Office license gets a new email address, which all route to a special mailbox I've setup.
Like with John, I suddenly couldn't log in one day and there was a message about how this email address - this unique-used-only-one-time-on-the-planet email address was sending spam??!! Seriously?
Something is brewing over there and I don't like the pattern that the data points are starting to reveal.