I'm going to play a little bit of devil's advocate here, because I think your comment has the beginning of a good conversation.

>* Email providers have zero responsibility for filtering information

Not necessarily true. Spam filtering is an expected feature nowadays, I would say, bit you're right that e-mail providers aren't required to do that sort of thing. Still, known spam IPs are regularly blacklisted from major e-mail providers.

>* Email users form their own networks dynamically through actual relationships rather than some artificial structure of "followers"/"friends"

Mailing lists and newsletters are more like a social network than typical e-mail. But the fundamental difference isn't how users form their networks, it's how the networks are structured: on Twitter, I take a message and broadcast it to everyone and Twitter puts ads around it, and other people see it even if they don't "know" me. The equivalency is e-mail:Twitter DMs :: massive mailing list:Twitter.

>* Email users never get banned from sending email (although maybe in some cases an individual service may ban extreme abusers)

You can get booted from a mailing list, and other users can block you, but more or less it's true that Microsoft won't ban me from Exchange servers. And that's kind of the thing, e-mail isn't centrally hosted. Mastodon has attempted this with a Twitterlike social media platform.

>* Email's operation is totally independent of irrational mobs/corporate interests/whatever

E-mails killer feature, for sure.

>In short, I think the core problem with this situation is in your first line: social media has become a "main communication provider" and it's fundamentally unfit for purpose. Email and other protocol-based communication independent of platforms (IRC is another good example) is still the best solution for a primary communication provider.

Agree, and I think this is why lots of online communities treat traditional social media (Facebook, Twitter, Instagram, Snapchat) like little broadcast hubs where you basically show the world your best in little snippits. On Twitter: post a joke or a funny thread; on Instagram: post your outfit, or your view, or your dinner; on Facebook: post your graduation and family photos; on Snapchat(/Instagram Stories): post about the little things you encounter throughout your day. Once you deviate from this framework – e.g. posting "politically incorrect" or intentionally controversial content – then you are subverting and ultimately trying to dismantle the cultural expectations for those platforms. The culturally accepted place for controversial content is in private conversations: Discord, WhatsApp, iMessage, e-mail, phone calls. Because then your victims have a way to defend themselves.

The argument over "free speech" on Twitter is undertaken, almost 100% of the time, in bad faith. Twitter is a bullhorn, P2P messaging is where free speech is truly (and should always be) protected. If you take a megaphone into Union Square, and start hurling slurs and threats at people, you will be removed or arrested the same way you'd be removed from Twitter for the same actions.

On Twitter you're not talking to "somebody", you're talking to "everybody". If you want to be controversial, take it to private conversation. Causing people pain in public isn't related at all to free speech – it's just another form of harassment.