a messaging protocol

Your URL is the address.

Posta is a wire protocol that lets one URL send a verified message to another. You publish a small JSON file at your URL (say taylor.posta.no) listing your public keys. The sender HTTPS POSTs a signed envelope to that URL; the receiver checks the Ed25519 signature against the key you published. That is the whole protocol.

A URL is the only address you can easily own. It is the same primitive as your homepage: a name in DNS pointing at infrastructure you control. You don't borrow it from a platform, and it doesn't live as a row in someone else's database. The TLS certificate that proves taylor.posta.no is taylor.posta.no is the same trust anchor for the keys it publishes.

The receiver fetches those keys with an HTTP GET on the URL itself, which returns an actor doc: a small JSON file listing the keys currently authoritative there. A POST to the same URL delivers a message: a JSON body of whatever payload, plus an Ed25519 signature over its raw bytes. Because the actor doc and the inbox live at the same URL, the URL is the identity rather than an address attached to one.

The wire format is fixed at proposal 1: under thirty kilobytes of normative Markdown plus a set of conformance test vectors. Any language with an HTTP client and Ed25519 can implement a Posta participant in well under a hundred lines. The Go reference is one of those; read the spec and build another.