best response confirmed by Sylvester- (Occasional Contributor)

@Sylvester- I think it really depends on HOW the other Federated domains have their configuration set.

The first link doesn't exactly mention IF the External participants can see your e-mail.  However, If they already have it then they would be able to send a Chat request or add you as a Contact. I suppose a malicious user in a Federated Domain could spray your users if they had a list of e-mails but that is very low chance if the Federated domain is trustworthy.

I did find another article at the very bottom of the first doc that tries to explain External versus Guest.