As Microsoft are no longer recommending the use of that server name, I can only suggest you change it to something they do recommend.

Incoming Server Type POP3 Server name outlook.office365.com on port 995 Using SSL/TLS connection security and the authentication method of OAuth2

Thanks Matt for getting back to me. Understanding and implementing your suggestion is a little over my head technically but at a more simplistic level I wonder why Microsoft would change something that I've successfully used for years and not tell me about it with specific instruction as to how to proceed. Also find it curious that the the error message indicates a problem with the outlook.com server when the email server I'm set up to access is a earthlink server (sprynet.com) which has nothing to do with Microsoft or outlook.com. (The only reason I have a Outlook account, which I almost never use, is because apparently you can't own a Windows computer without one). Also interesting that after getting the error message saying login failed my earthlink messages are still downloaded (after a slight delay). And when I send an email I get no error message. Causes me to wonder if the problem is a bug or some recently corrupted code in the Thunderbird software.