PSA: If you're using a #Hotmail or #Outlook email with #Thunderbird and getting "unusual sign-in activity" alerts, you're not alone.

This appears to be an issue on Microsoft's end, and is being actively investigated.

This post on the Microsoft Answers forum has detailed and constantly updated info. https://answers.microsoft.com/en-us/outlook_com/forum/all/outlook-mail-account-unusual-activity/6cfb226f-8cd7-41ae-bd0a-a80d100af543?page=2