Add NIP-M4: Email

M4.md

@@ -0,0 +1,28 @@
+NIP-M4
+======
+
+Email
+-----
+
+`draft` `optional`
+
+Email over Nostr: a standard [RFC 2822](https://www.rfc-editor.org/rfc/rfc2822) MIME message carried in the `content` of a `kind:322` event, using Nostr as transport instead of SMTP. A Nostr user's address is `<npub>@nostr`.
+
+## Delivery
+
+- **Private (default):** build the `kind:322` as an unsigned rumor and [NIP-59](59.md) gift-wrap it to each recipient and to self, published to the recipient's [NIP-17](17.md) `kind:10050` DM relays. The rumor MAY be signed to prove authorship to third parties.
+- **Public:** publish a signed `kind:322` directly to [NIP-65](65.md) relays. BCC recipients are notified privately with a gift-wrapped copy carrying a `["public-ref", "<event-id>", "<relay-url>"]` tag pointing to the public event.
+
+```json
+{
+  "kind": 322,
+  "content": "From: npub1alice...@nostr\r\nTo: npub1bob...@nostr\r\nSubject: Hi\r\n\r\nHey Bob"
+}
+```
+
+## Bridges (legacy email)
+
+A bridge relays to and from legacy SMTP email and is discovered via [NIP-05](05.md) at `_smtp@<domain>`.
+
+- **Outbound (Nostr → SMTP):** gift-wrap the `kind:322` to the bridge's pubkey with `mail-from` and `rcpt-to` tags (multiple `rcpt-to` tags for several recipients). The bridge delivers via SMTP.
+- **Inbound (SMTP → Nostr):** the bridge builds the `kind:322` rumor on the sender's behalf and gift-wraps it to the recipient, setting `mail-from` to the legacy sender address so the recipient can tell it apart from a direct Nostr email.

README.md

@@ -116,6 +116,7 @@ NIPs listed here are not a protocol checklist. Nothing forces any software to im
 - [NIP-CC: Geocaching](CC.md)
 - ~~[NIP-EE: E2EE Messaging using MLS Protocol](EE.md) --- **unrecommended**: superseded by the [Marmot Protocol](https://github.com/marmot-protocol/marmot)~~
 - [NIP-F4: Podcasts](F4.md)
+- [NIP-M4: Email](M4.md)
 
 ## Event Kinds
 
@@ -155,6 +156,7 @@ This table is not exhaustive. For a machine-readable registry of all known event
 | `62`          | Request to Vanish               | [62](62.md)                            |
 | `64`          | Chess (PGN)                     | [64](64.md)                            |
 | `78`          | Application-specific Data       | [78](78.md)                            |
+| `322`         | Email                           | [M4](M4.md)                            |
 | `443`         | KeyPackage                      | [Marmot](marmot)                       |
 | `444`         | Welcome Message                 | [Marmot](marmot)                       |
 | `445`         | Group Event                     | [Marmot](marmot)                       |