Aggressive LMTP header cleanup (#816)

This will remove all headers possible during LMTP delivery, except:

- From: required or core does not process the message correctly.
  Also required for cleartext compatibility.
- Message-Id: required for clients to know which messages have been
  downloaded
- Chat-Is-Post-Message: is required for our attachment previews
- Content-Type: required
- For Cleartext compability: To, CC, In-Reply-To, References, Subject,
  and Date
- For Chatmail future expansion, allow Chat-*
- Permit the entire Secure-Join* namespace

Co-authored-by: holger krekel  <holger@merlinux.eu>

cmdeploy/src/cmdeploy/postfix/lmtp_header_cleanup

@@ -1,3 +1,23 @@
-/^DKIM-Signature:/            IGNORE
-/^Authentication-Results:/            IGNORE
-/^Received:/            IGNORE
+# List of headers for incoming messages 
+# that must be retained for functionality and compatibility reasons 
+/^From:/            DUNNO
+/^Message-Id:/      DUNNO
+/^Chat-/            DUNNO
+/^Content-Type:/    DUNNO
+
+# For receiving clear-text messages (still supported in May 2026)
+/^Subject:/         DUNNO
+/^Date:/            DUNNO
+/^To:/              DUNNO
+/^CC:/              DUNNO
+/^References:/      DUNNO
+/^In-Reply-To:/     DUNNO
+
+# Senders might support Autocrypt 1 but not RFC9788 (Header Protection) 
+/^Autocrypt:/       DUNNO
+
+# SecureJoin V2 protocol headers (for backward compatibility) 
+/^Secure-Join/      DUNNO
+
+# Ignore all other headers
+/.*/                IGNORE