Well to be fair it does say this in the FAQ section on their product page:

Q: Which email provider(s) will Notion Mail be compatible with?

A: Notion Mail will integrate seamlessly with Google and Gmail accounts at initial launch.

I assume this is because they are using Google’s APIs to access your email and just add some extra bells and whistles on top of that.

Makes it utterly worthless to me, though. I’m trying to slowly REMOVE Google and Gmail from my life, not get locked in on using it.