fix: ensure message content is not modified by header filter

Due to an oversight, one of the header filters has potentially modified
the message body, leading to broken cryptographic signatures.

Switch to the Message-Id instead of the Content-Type header, which is
usually only present in the global header.

Signed-off-by: Moritz Poldrack <git@moritz.sh>
Co-Authored-by: Georg Lauterbach <44545919+georglauterbach@users.noreply.github.com>
Reported-by: Cole Young <cole@young.sh>
Fixes: 009237cc ("chore: Prevent Microsoft MUAs from sending reactions (#4120)")

CHANGELOG.md

@@ -6,6 +6,11 @@ All notable changes to this project will be documented in this file. The format
 
 > **Note**: Changes and additions listed here are contained in the `:edge` image tag. These changes may not be as stable as released changes.
 
+### Fixes
+
+- **Postfix**
+  - Avoid modifying the message body when filtering sender headers. This regression was introduced from [#4120](https://github.com/docker-mailserver/docker-mailserver/pull/4120) as part of DMS v15.0.0 ([#4429](https://github.com/docker-mailserver/docker-mailserver/pull/4429))
+
 ## [v15.0.1](https://github.com/docker-mailserver/docker-mailserver/releases/tag/v15.0.1)
 
 ### Added

target/postfix/sender_header_filter.pcre

@@ -8,4 +8,5 @@
 /^\s*X-Mailer/                      IGNORE
 /^\s*X-Originating-IP/              IGNORE
 /^\s*Received: from.*127.0.0.1/     IGNORE
-/^Content-Type:/i                   PREPEND X-MS-Reactions: disallow
+/^\s*X-MS-Reactions:/               IGNORE
+/^\s*Message-Id:/i                  PREPEND X-MS-Reactions: disallow

test/tests/parallel/set3/container_configuration/hostname.bats

@@ -234,8 +234,9 @@ function _should_have_correct_mail_headers() {
   # but Amavis is changing that. It also changes protocol from SMTP to ESMTP.
   assert_line --index 7 --partial 'Received: from localhost (localhost [127.0.0.1])'
   assert_line --index 8 --partial "by ${EXPECTED_FQDN} (Postfix) with ESMTP id"
-  assert_line --index 14 --partial 'Message-Id:'
-  assert_line --index 14 --partial "@${EXPECTED_FQDN}>"
+  assert_line --index 14 'X-MS-Reactions: disallow'
+  assert_line --index 15 --partial 'Message-Id:'
+  assert_line --index 15 --partial "@${EXPECTED_FQDN}>"
 
   # Mail contents example:
   #