Replace subject with [...] for outgoing mail

`authclean` cleanup server is used by
reinjecting smtpd running on localhost:10025 by default.
It runs after filtermail
and currently removes `Received` header
to avoid leaking IP address.
Can as well be used to replace `Subject` lines
with `Subject: [...]`.
If there are multiple `Subject` lines,
all of them should be replaced.

This allows us to avoid dealing with
localized subjects, including SecureJoin
messages `vc-request` and `vg-request`
which can have Subject lines like
Subject: =?utf-8?q?Nachricht_von_nrn178fi4=40nine=2Etestrun=2Eorg?=

CHANGELOG.md

@@ -11,6 +11,9 @@
 - improve secure-join message detection
   ([#473](https://github.com/deltachat/chatmail/pull/473))
 
+- replace `Subject` with `[...]` for all outgoing mails.
+  ([#481](https://github.com/deltachat/chatmail/pull/481))
+
 ## 1.5.0 2024-12-20
 
 - cmdeploy dns: always show recommended DNS records

chatmaild/src/chatmaild/filtermail.py

@@ -12,7 +12,6 @@
 
 from aiosmtpd.controller import Controller
 
-from .common_encrypted_subjects import common_encrypted_subjects
 from .config import read_config
 
 
@@ -128,8 +127,6 @@ def check_encrypted(message):
     """
     if not message.is_multipart():
         return False
-    if message.get("subject") not in common_encrypted_subjects:
-        return False
     if message.get_content_type() != "multipart/encrypted":
         return False
     parts_count = 0

chatmaild/src/chatmaild/tests/test_filtermail.py

@@ -5,7 +5,6 @@
     SendRateLimiter,
     check_armored_payload,
     check_encrypted,
-    common_encrypted_subjects,
     is_securejoin,
 )
 
@@ -71,18 +70,13 @@ def test_filtermail_securejoin_detection(maildata):
 
 
 def test_filtermail_encryption_detection(maildata):
-    for subject in common_encrypted_subjects:
-        msg = maildata(
-            "encrypted.eml",
-            from_addr="[email protected]",
-            to_addr="[email protected]",
-            subject=subject,
-        )
-        assert check_encrypted(msg)
-
-    # if the subject is not a known encrypted subject value, it is not considered ac-encrypted
-    msg.replace_header("Subject", "Click this link")
-    assert not check_encrypted(msg)
+    msg = maildata(
+        "encrypted.eml",
+        from_addr="[email protected]",
+        to_addr="[email protected]",
+        subject="Subject does not matter, will be replaced anyway",
+    )
+    assert check_encrypted(msg)
 
 
 def test_filtermail_no_literal_packets(maildata):

cmdeploy/src/cmdeploy/postfix/submission_header_cleanup

@@ -2,3 +2,4 @@
 /^X-Originating-IP:/    IGNORE
 /^X-Mailer:/            IGNORE
 /^User-Agent:/          IGNORE
+/^Subject:/             REPLACE Subject: [...]