Hi @Angus Kitchin ,
Welcome to our forum. Have you tried to test with the method of register key as you mentioned?
If so, for your issue, I have tried to test on my Outlook 365, I would also see a similar tips when I open Outlook like below:
And I found the tip would disappear when I turn off the "optional connected experience", so it's suggested to turn off this option to check if it could also work for you (File>Options>Trust center>Trust center settings>Privacy option>Privacy settings>turn off this option)
Hope that would also be helpful to you.
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