> If my mail server is banning mail from Yahoo, I can't communicate with my grandparents and I stop using that mail server. Enough people do that and the mail server has no users.

Why are your grandparents using Yahoo instead of your mail server?

> Have you taken your own advice here and blocked @yahoo.com incoming? Is it inconvenient? Is it more inconvenient than the two-step process of setting up a Gmail account?

I haven't had any correspondence from anyone who uses @yahoo.com. Or, if I have, they haven't complained about me not receiving their email. Or, if they have, their complaint was also not received in which case it doesn't exactly matter. If it did matter then I would address it then. And, importantly, it also means there's another (less noisy) communication medium available already.

> Why are your grandparents using Yahoo instead of your mail server?

Because internet systems are one part technology and one part social. My grandparents already have Yahoo accounts and are unwilling to change that.

And if your solution to interoperating with Yahoo servers is "I don't have anyone to talk to using Yahoo servers," then I'm afraid it sounds like you're trying to solve a problem other than the one email is designed to solve.