As you know, Exchange administrators have been able to flag external sender for few weeks already (see https://t.co/144cAWA42F) but this was applicable only to Outlook mobile.
Well, good news, if you have already configured it for your Outlook mobile users, this also now applies to Outlook client, including Outlook Web Access (aka OWA, aka Outlook on the Web).

If you have not yet enabled this feature, this is a good time to do it.

For OWA, you have nothing to do. For Outlook clients on Windows you will need to run the latest updates made available in May 2021 (Fast Insider as first target) and for Outlook client on MacOS you will need to run the version 16.47.

Outlook Client	Outlook Web Access