Most people I know use gmail (invluding, it must be said, my two friends in Germany).
Fwiw, i just looked up a bunch of my German contacts, and didn’t find any Gmail address. I did accidentally include Danish and Swedish friends in that search, who all had gmail addresses. 
I don’t have many international contacts, and only one American, but I looked up his email address, and alas, it’s Gmail!
Six days in, I’ve got a winner. I’ve only had to affirmatively protect 3 gmail senders who weren’t sending book-and-author spam.
Ironically, I think that if you had a Gmail account for your author info, this stuff probably would have been blocked by Gmail’s spam filters before it got to your email client.
Not all of it. I get some author-directed spam of this sort in gmail.