Why not just run Postfix+Dovecot? If everyone runs a single-user server, it becomes a P2P network. Also, it’dn’t depend on Tor, which is an external dependency.

Also, what role is Tor playing in privacy? If the chatting parties are cryptographically verifed, only message content remains private…

Postfix+Dovecot: technically doable, but "everyone self-hosts email" has been the theoretical answer for 25 years and never happened. You need a static IP, domain, reverse DNS, SPF, DKIM, DMARC, and even then Gmail drops half your mails in spam because small senders have no reputation. Full sysadmin job just to message your aunt. PeerBox is built for non-technical users to install in 5 minutes and have it just work. No domain, no DNS, no reputation game. Role of Tor: it's not there to encrypt content (HMAC and the encrypted vault handle that). Tor hides the metadata. Without it, your ISP sees machine A talking to machine B, when, how often, how much data. Content is the easy part to encrypt. Who-talks-to-whom is the hard part, and hidden services solve it cleanly. No IPs exposed, no direct connection, no social graph for a passive observer to build. And Tor being an "external dependency" is a feature. Better to lean on a mature, audited network than roll our own obfuscation layer :)