I think he means that it might have an effect on other people's priority inbox. It could be that if an incoming E-Mail has another recipient, GMail thinks this E-Mail might be more important and place it in the priority inbox.

Yes. I misunderstood.

I know for a fact that most of those contacted do not use Gmail. Only one confirmedly does.

There might be other corporate non-public solutions that do that, on the other hand.