Maybe calling an instance a fediverse providors might help bridge the gap?

So email can work with multiple providers because they all speak the same federated language. Gmail can talk to yahoo can talk to outlook can talk to thunderbird etc etc

And Mastadon can talk to Lemmy can talk to peertube can talk to threads and on and on.

They can’t talk to each other normally because they speak different languages (email protocol and the fediverse protocol).