feat: emit a signed Reply-To by default, --anonymous to omit

README.md

@@ -70,6 +70,8 @@ uv run smolmail.py list
 uv run smolmail.py read <id>
 ```
 
+Sent mail carries the sender's full `smol://` address in a signed `Reply-To` field (SPEC.md §5.7), so a first-time recipient can answer without an out-of-band channel; pass `--anonymous` to omit it.
+
 `import` adds a contact from a `smol://` address without trusting any server, `contacts` lists known keys and how each was learned, and `rotate` replaces the identity with a signed certificate your contacts accept automatically.
 
 Mail is stored sealed and opened on demand, so the local database holds no plaintext. Superseded keys are retained after a rotation, because mail already addressed to them is readable with nothing else.

smolmail.py

@@ -680,6 +680,9 @@ def cmd_send(args: argparse.Namespace, store: Store) -> int:
         if not sep or not FM_KEY.match(key.strip()):
             raise SmolError(f"{raw!r} is not a valid `Key: value` header")
         fields.append((key.strip(), value.strip()))
+    # §5.7: a signed reply address lets a first-time recipient answer us.
+    if (account := store.account()) is not None and not args.anonymous:
+        fields.append(("Reply-To", account.uri(me.pk)))
 
     body = build_frontmatter(fields, text).encode()
     envelope = seal(me, recipient, body, pad=not args.no_pad)
@@ -828,6 +831,8 @@ COMMANDS = [
       (("--body",), {"help": "message text; read from stdin when omitted"}),
       (("--header",), {"action": "append", "metavar": "KEY:VALUE",
                        "help": "extra frontmatter field; repeatable"}),
+      (("--anonymous",), {"action": "store_true",
+                          "help": "omit the Reply-To field carrying this address (SPEC.md §5.7)"}),
       (("--no-pad",), {"action": "store_true", "help": "do not pad to 1 KiB"})]),
     ("fetch", cmd_fetch, "retrieve, verify and acknowledge mail",
      [(("--keep",), {"action": "store_true", "help": "do not delete from the server"})]),