What's the authentication method, normal password or OAuth2? O365 accounts seem to be either very easy or impossibly complex to set up in mail clients other than Outlook.

What's the authentication method, normal password or OAuth2? O365 accounts seem to be either very easy or impossibly complex to set up in mail clients other than Outlook.

Set as normal password, although I have tried OAuth2, and endless other permutations. This was setup in April 2017 and has worked until a few days ago.

What changed a few days ago? See https://support.mozilla.org/en-US/questions/1360016

If you can't get the O365 smtp to work, try adding a regular consumer outlook.com account or gmail account, and set the O365 account to send on the outlook.com or gmail smtp. This is often recommended for users with a poor or non-secure smtp. The O365 can be added as an alias in outlook.com webmail settings or a 'send as' account in gmail settings, so the recipient still sees messages as being sent from the O365 account.

A consumer outlook.com account has the same server settings as O365, except the authentication method is 'normal password'.

If you can't get the O365 smtp to work, try adding a regular consumer outlook.com account or gmail account, and set the O365 account to send on the outlook.com or gmail smtp. This is often recommended for users with a poor or non-secure smtp. The O365 can be added as an alias in outlook.com webmail settings or a 'send as' account in gmail settings, so the recipient still sees messages as being sent from the O365 account. A consumer outlook.com account has the same server settings as O365, except the authentication method is 'normal password'.

The original settings was to smtp-mail.outlook.com, and now changed to smtp.office365.com, with a lot of changes as part of the trouble shooting process. So, I'm not seeing a difference between the two. In the web Outlook the suggested setting for sync are Office365.com. (Hoping I've understood your suggestion correctly).

The consumer/personal Hotmail/outlook.com accounts have the same server settings as the O365 accounts, the difference being the User Name for the former are user@hotmail.com or user@outlook.com, while the latter are user@domain.com, and the O365 accounts use OAuth2 authentication.

I don't have an O365 account to test, but the option of setting an smtp that is different from the normal one is available, e.g. set an account user@myisp.com to send through smtp.gmail.com instead of smtp.myisp.com.

It appears the problem, and solution, lie with Microsoft. The Microsoft account was originally setup in 2017 using what was then called Outlook Premium and allowed the use of personalised email, i.e. using your own domain. Microsoft dropped support for this service in February 2021, without direct notice, but Thunderbird continued to work until early December 2021. It is not possible to setup the desktop version of Outlook as an alternative, but the web version of the Outlook still works (but for now long?). Note the continuing requirement for the custom email is essential.

The Microsoft solutions are 1) use the business version of 365 where the use of the customers domain is standard, but at much higher cost than 365 Family. 2) Continue to use 365 Family and move the domain registration to GoDaddy, the only domain supported by Microsoft for the personalised email (but for how long?). It would have been nice to have received some notification from Microsoft, but there you go.

My thanks to sfhowes for their help.