This is the issue I ran into that I referenced in my earlier post about having to jump through a bunch of hoops. I am sorry I did not document what exactly I did but I had to install Exchange Powershell in order to hydrate. Of course there were a couple of intermediate powershell commands because nothing is that easy. You can try this link

https://theadmin365.com/how-to-enable-organization-customization-in-office-365/

Thanks for that danielsan - but as it only buys me a few weeks respite I will not enter another headbanging contest with Microsoft.

My workround is now to use Outlook Web Access (OWA) via a browser to receive my mail. The main disadvantage is it doesn't immediately show me the email alias address on the message.

For outgoing mail OWA doesn't allow you to use the registered alias addresses.

However - Pegasus SMTP configured with a password (possibly only an appPassword??) can still send mail with my registered alias sender addresses. Microsoft have had to leave SMTP off Oauth 2 as there are too many user devices that cannot be reconfigured to send emails notifications.

Addendum:
OWA also seems to have no way to store messages locally on my PC. So I am manually forwarding them to my gmail account - for which Pegasus Oauth2 workround is currently viable. If that goes off - then I wil have to use my webmaster site supplier's email - which is currently not showing any sign of an Oauth 2 transition. But then again - I never heard a peep about the MS Oauth2 until I saw an El Reg article in September about the shut-off coming..