Using older versions of Outlook for Mac in business environments
Indeed, the link is to the Microsoft page explaining Microsoft's account activity policy wherein it says
Microsoft account activity policy: Under the MSA, you must use your Microsoft account to keep it “active.” Microsoft reserves the right to close all “inactive” Microsoft accounts. Except as provided below, you must sign in to your Microsoft account at least once in a two-year period to keep your account active. If you don’t sign in during this time, Microsoft will consider your account to be inactive.
When one combines this with your statement "I regularly log in to this account and indeed had done earlier that day.", there is only ONE possible conclusion, IHMO, and that is this email IS A PHISH!!!! I repeat that I get several of these emails a week, ALL are SCAMS.