Twitter is pretty abusive. And email used to be even worse.
It didn't get decent until it people started flocking toward centralized solutions like GMail, which (coincidentally or not) did a pretty decent job of reining in spam and viruses.
It's federated but certainly not completely decentralized. Spam filters in a way have become "a protection (even if flawed)". With email, you can still send an email to anyone but if you send it from a personal server versus gmail, it's much less likely to be read.
That's not been my experience at all. I've been running my own email server for years and I've yet to hear about an email being blocked. I know many other people who run their own email server just fine.
It didn't get decent until it people started flocking toward centralized solutions like GMail, which (coincidentally or not) did a pretty decent job of reining in spam and viruses.