The MS letter mentioned more than authentication: you need outlook.office365.com (SSSL/TLS and 993 for IMAP) as incoming server and smtp.office365.com (STARTTLS and port 587) as outgoing server and you do not need to remove and add back the account. If you are having trouble, please post screenshots of incoming and outgoing servers

Removing and re-adding is certainly the easiest way. As for loosing your e-mails it depends on how it's set-up. If it's using IMAP the e-mails never leave MicroSoft's server, unless you delete them, and Thunderbird is only a tool to view them. If it's set to use POP3 then they are downloaded to your computer and the originals probably deleted. If there's uncertainty it's best to keep this method as a last resort. As an aside if your e-mails are on your computer you really should look into how to make backup copies as hard disks can fail and you'd loose the lot.

I'm on HotMail and here are the settings that work fine for me:—

Server Name: outlook.office365.com Port: 993 User Name: <my name>@hotmail.com Connection security: SSL/TLS Authentication method: OAuth2

Now click 'Outgoing Server (SMTP) in the left hand column. Double click the Outlook.com entry and enter these details:—

Server Name: smtp.office365.com Port: 587 Connection security: STARTTLS Authentication method: OAuth2 User Name: <your e-mail address>

I am assuming that you are using the current, or at least a fairly recent version of Thunderbird. Old ones don't support OAuth2.