follow link2xt advise and don't check subject/body at all -- turns out there were no tests anyway.

chatmaild/src/chatmaild/filtermail.py

@@ -34,6 +34,34 @@ def check_encrypted(message):
     return True
 
 
+def check_mdn(message, envelope):
+    if len(envelope.rcpt_tos) != 1:
+        return False
+
+    for name in ["auto-submitted", "chat-version"]:
+        if not message.get(name):
+            return False
+
+    if message.get_content_type() != "multipart/report":
+        return False
+
+    body = message.get_body()
+    if body.get_content_type() != "text/plain":
+        return False
+
+    if list(body.iter_attachments()) or list(body.iter_parts()):
+        return False
+
+    # even with all mime-structural checks an attacker
+    # could try to abuse the subject or body to contain links or other
+    # annoyance -- we skip on checking subject/body for now as Delta Chat
+    # should evolve to create E2E-encrypted read receipts anyway.
+    # and then MDNs are just encrypted mail and can pass the border
+    # to other instances.
+
+    return True
+
+
 class SMTPController(Controller):
     def factory(self):
         return SMTP(self.handler, **self.SMTP_kwargs)
@@ -82,6 +110,9 @@ def check_DATA(envelope):
     if envelope.mail_from.lower() != from_addr.lower():
         return f"500 Invalid FROM <{from_addr!r}> for <{envelope.mail_from!r}>"
 
+    if not mail_encrypted and check_mdn(message, envelope):
+        return
+
     envelope_from_domain = from_addr.split("@").pop()
     for recipient in envelope.rcpt_tos:
         if envelope.mail_from == recipient:

tests/chatmaild/test_filtermail.py

@@ -1,4 +1,4 @@
-from chatmaild.filtermail import check_encrypted, check_DATA, SendRateLimiter
+from chatmaild.filtermail import check_encrypted, check_DATA, SendRateLimiter, check_mdn
 import pytest
 
 
@@ -41,8 +41,33 @@ def test_filtermail_encryption_detection(maildata):
     assert not check_encrypted(msg)
 
 
-def test_filtermail_mdn_is_not_encrypted(maildata):
-    assert not check_encrypted(maildata("mdn.eml"))
+def test_filtermail_is_mdn(maildata, gencreds):
+    from_addr = gencreds()[0]
+    to_addr = gencreds()[0] + ".other"
+    msg = maildata("mdn.eml", from_addr, to_addr)
+
+    class env:
+        mail_from = from_addr
+        rcpt_tos = [to_addr]
+        content = msg.as_bytes()
+
+    assert check_mdn(msg, env)
+    print(msg.as_string())
+    assert not check_DATA(env)
+
+
+def test_filtermail_to_multiple_recipients_no_mdn(maildata, gencreds):
+    from_addr = gencreds()[0]
+    to_addr = gencreds()[0] + ".other"
+    thirdaddr = gencreds()[0]
+    msg = maildata("mdn.eml", from_addr, to_addr)
+
+    class env:
+        mail_from = from_addr
+        rcpt_tos = [to_addr, thirdaddr]
+        content = msg.as_bytes()
+
+    assert not check_mdn(msg, env)
 
 
 def test_send_rate_limiter():

tests/conftest.py

@@ -290,7 +290,7 @@ class Remote:
 def maildata(request, gencreds):
     datadir = conftestdir.joinpath("mail-data")
 
-    def maildata(name, parsed=True, from_addr=None, to_addr=None):
+    def maildata(name, from_addr=None, to_addr=None):
         if from_addr is None:
             from_addr = gencreds()[0]
         if to_addr is None:

tests/mail-data/mdn.eml

@@ -1,6 +1,6 @@
 Subject: Message opened
-From: <barbaz@c2.testrun.org>
-To: <foobar@c2.testrun.org>
+From: <{from_addr}>
+To: <{to_addr}>
 Date: Sun, 15 Oct 2023 16:43:25 +0000
 Message-ID: <Mr.78MWtlV7RAi.goCFzBhCYfy@c2.testrun.org>
 Auto-Submitted: auto-replied

tests/online/test_2_deltachat.py

@@ -1,3 +1,4 @@
+import time
 import random
 import pytest
 
@@ -81,3 +82,29 @@ class TestEndToEndDeltaChat:
         ch = ac2.qr_setup_contact(qr)
         assert ch.id >= 10
         ac1._evtracker.wait_securejoin_inviter_progress(1000)
+
+    def test_read_receipts_between_instances(self, cmfactory, lp, maildomain2):
+        ac1 = cmfactory.new_online_configuring_account(cache=False)
+        cmfactory.switch_maildomain(maildomain2)
+        ac2 = cmfactory.new_online_configuring_account(cache=False)
+        cmfactory.bring_accounts_online()
+
+        lp.sec("setup encrypted comms between ac1 and ac2 on different instances")
+        qr = ac1.get_setup_contact_qr()
+        ch = ac2.qr_setup_contact(qr)
+        msg = ac2.wait_next_incoming_message()
+        assert "verified" in msg.text
+
+        lp.sec("ac1 sends a message and ac2 marks it as seen")
+        chat = ac1.create_chat(ac2)
+        msg = chat.send_text("hi")
+        m = ac2.wait_next_incoming_message()
+        m.mark_seen()
+        # we can only indirectly wait for mark-seen to cause an smtp-error
+        lp.sec("try to wait for markseen to complete and check error states")
+        deadline = time.time() + 3.1
+        while time.time() < deadline:
+            msgs = m.chat.get_messages()
+            for msg in msgs:
+                assert "error" not in m.get_message_info()
+            time.sleep(1)