release: 2.4.0: an application signs only as itself

from-header-binding (docs/plans/from-header-binding.md, 9/9), at the owner's
word (2026-10-12, "Да").

  - A message's From: line is held to what its envelope sender is held to:
    any address of the application's domain, or the addresses of its list,
    with or without a +extension. Anything else is refused with 550 5.7.1 -
    another registered domain, the server's own name, a domain this server
    does not sign for. Until now an application could have mail signed with
    another domain's key by writing that domain into From:.
  - The check is the panel's, first in the milter chain and strict: while
    the panel does not answer, Postfix defers mail.
  - Server > Health counts what that costs - the panel's downtime under a
    running Postfix and the submissions turned away - and is laid out anew,
    boxes numbered by importance.
  - A refused row of the Outbound log says why in its tag.
  - Migration 0006.

Upgrading: mail whose From: is not the application's own stops being
accepted - another of your domains, an address off an application's list, a
visitor's address a form puts there. The guide says what to do about each.
A stopped panel now stops submission.

Not done, written into the plan at the owner's word: the Delivery log box on
a refused message's page, the log's status filter, "messages" that are
connections, and what was reasoned about rather than run - OpenDKIM's own
parser above all.

Cut: CHANGELOG 2.4.0 written (nothing had been put under [Unreleased]
while building); image pin 2.4.0 in compose, README, guide; version table,
roadmap, plan status. security.md: a malformed packet on a milter socket,
as an accepted risk. No tag: the owner tags when he decides to, so there is
no image of this version until then.

On the test server, on c817f08: go vet ./... ok, go test ./... ok
(23 packages), make e2e ok. This commit changes documents and the pin only.

Full detail: CHANGELOG.md (2.4.0).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

deploy/docker-compose.yml

@@ -21,7 +21,7 @@
 
 services:
   selfpost:
-    image: ghcr.io/mixeme/selfpost:2.3.0
+    image: ghcr.io/mixeme/selfpost:2.4.0
     restart: unless-stopped
     environment:
       SELFPOST_HOSTNAME: "${SELFPOST_HOSTNAME:?set the mail/panel hostname, e.g. mail.example.com}"