Thunderbird mail on macOS
Download and install Thunderbird mail from Thunderbird.net
click on "Manual config" and fill in field as shown below
click done and you will be prompted with your SSO login and MFA.
If OAuth2 is not in the two drop-down lists of authentication types, put in all the details, and hit "Re-test". Then OAuth2 should appear as a choice in those two drop-downs (this is a recent change as of 7th October 2021: please contact itsupport@physics.ox.ac.uk if you have problems with this).
You must also set your Email Address to firstname.lastname@physics.ox.ac.uk instead of abcd1234@OX.AC.UK or replies to you will bounce.
Go to the menu button (three horizontal lines) and choose Account Settings. The topmost item should be selected, and will have the fields "Account Name", "Your Name", "Email Address" and others. Edit Email Address as described above.