You guys are still thinking in terms of the phone system with publicly accessible numbers!
In future networks you’ll have to have an invitation path from the user, and if it gets abused you just mute a subpath so those people’s invites don’t result in auto-accepting messages. Simple!
A -> B -> C -> D
D attempts to send a message to A’s mailbox
A’s mailbox automatically accepts the message
If too many messages were sent from the subtree of invitations of B or C, just mute that branch.
Then the others have to jump through hoops like proof of work or pay crypto to be whitelisted and start a conversation to you.
Fixes all SPAM.
You can make this compatible with an email gateway where the invitation is added as an email alias such as “foobar@dontspamme.com” and then emails to and from “foobar@“ would be proxied as messages to the actual non-email system I described, where foobar was the gateway corresponding to the “path A -> B”. It was compromised? Don’t accept emails from any new unknown email addresses sending to that endpoint without jumping through hoops.