Have no idea to whom I was chatting a week or so ago, but outlook.office365.com was recommended to me as the incoming server setting for Outlook.com. Set TB that way and used it for a week or so. TODAY got a "failure to authenticate" error from TB (have had many such errors before). Went directly to Copilot, as I thought it would be the most authoritarian place to clear up Microsoft problems. It provided the informaiton below: Official Outlook.com Thunderbird Incoming '''Server Settings (2026)''' IMAP (recommended) Settings Server name: imap-mail.outlook.com Port: 993 Security: SSL/TLS Authentication; OAuth2 Username; Your full Outlook.com email address Why this is the correct server Outlook.com, Hotmail, Live, and MSN accounts do not use outlook.office365.com . Set outlook account incoming server to imap-mail.outlook.com and reset TB Immediately got: Mozilla Thunderbird Thunderbird Authentication failure while connecting to server imap-mail.outlook.com. Can someone please tell what's going on?