HI.
I found this thread. Which provides details as well. Shows all the threads that have been started regarding this issue. He even tried enabling 2F authentication, and still has the issue. He has a support ticket as well.
This is clearly a MS deal.
https://answers.microsoft.com/en-us/outlook_com/forum/all/outlook-mail-account-unusual-activity/6cfb226f-8cd7-41ae-bd0a-a80d100af543
I even have a thread from 2015 with the exact same thing happening.
https://answers.microsoft.com/en-us/outlook_com/forum/all/microsoft-account-unusual-sign-in-activity-ip-is/893dd29e-ff5c-48b5-a7d6-4588d5e83fc6
A user managed to get this response (I guess back when you could call and get a live person.
:S >>>>Good day! I understand the situation you are in and let me do my best to help you. Please be informed that IP addresses from Microsoft are generated from when your account is accessed by our system to apply updates to the account. This is the system accessing the account, not any particular individual from Microsoft accessing the account. <<<<
IF this is true, we as users need to be made aware of this. And if it's going to flag as unusual activity, we need to be told whether to ignore it or verify it is us. It's frustrating that this keeps occurring and there is no help documentation on this, nor a MS generated email that goes out to users that explain the situation. This is really just poor customer service, IMO
I also found an article regarding deprecation of basic authentication. The article makes it sound like MS is trying to move users away from POP3 and IMAP imports, and force us into downloading their app on our mobile devices. But I will admit, I didn't completely understand this article:
https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/deprecation-of-basic-authentication-exchange-online