Let's design the successor to email on top of HTTP, fixing SMTP's 40-year flaws. The goal isn't to replace Gmail but to learn by swapping every element: sending, receiving, keys, and gateways. The only kept convention is user@domain addresses. The protocol, dubbed HMTP (Hypertext Mail Transfer Protocol), assembles existing, battle-tested technologies: WebFinger for discovery, ActivityPub for delivery, Ed25519 for signatures, HPKE for encryption, sigchains for key rotation, and JMAP for sync. Each piece is standardized and deployed at scale by millions.

Discovery uses a static `.well-known/hmtp` JSON document, enabling per-user delegation without DNS — like MX records but over HTTP. Identity survives key rotation via sigchains (as ATProto does) with domain control as fallback. Delivery is a POST to the recipient's inbox, queued on the sender's server with exponential backoff and idempotent retries via content-hash IDs. Messages are signed objects; the envelope stays visible for routing while the body is sealed with HPKE. Attachments are referenced by hash and URL, avoiding base64 bloat.

Anti-spam layers start with identity cost: signing requires a domain, imposing a sybil cost. The system never talks to classic providers; it only talks to itself, proving that a modern mail protocol can be a convention over HTTP, not a new server.