Comment by baron3dl

let's say this federation has three communities, a, b, and c. you run a, I run b, and my spammy friend runs c. C is my friend, and I'm going to keep allowing him to send me email, but you drop him, because he spammed you.

that's literally how email works today, unless the federation can supersede my authority as an operator to choose who may send/receive with me, at which point it'd stop being a federation anyway.