feat: DKIM-sign bounce messages (mainly "user does not exist")

This was originally based on Jagoda's https://github.com/chatmail/relay/pull/874
but then the postfix config was simplified, and it comes with a simpler and more robust test.

cmdeploy/src/cmdeploy/postfix/main.cf.j2

@@ -103,3 +103,11 @@ smtpd_peername_lookup = no
 default_transport = lmtp-filtermail:inet:[127.0.0.1]:{{ config.filtermail_lmtp_port_transport }}
 lmtp-filtermail_initial_destination_concurrency=10000
 lmtp-filtermail_destination_concurrency_limit=10000
+
+{% if not config.ipv4_relay %}
+# DKIM-sign locally generated mail (bounces, DSNs).
+# These bypass smtpd, so they need explicit milter configuration.
+non_smtpd_milters = unix:opendkim/opendkim.sock
+internal_mail_filter_classes = bounce
+milter_macro_daemon_name = ORIGINATING
+{% endif %}

cmdeploy/src/cmdeploy/tests/online/test_1_basic.py

@@ -194,6 +194,34 @@ def test_reject_missing_dkim(cmsetup, maildata, from_addr):
             s.sendmail(from_addr=from_addr, to_addrs=recipient.addr, msg=msg)
 
 
+def test_bounces_are_dkim_signed(cmsetup, cmsetup2, maildata, maildomain):
+    # we send a message to non-existant user and expect a bounce message
+    # which will only get through if the bounce message was DKIM-signed
+
+    if is_valid_ipv4(maildomain):
+        pytest.skip("DKIM is not configured on IPv4-only relays")
+
+    sender = cmsetup2.gen_users(1)[0]
+    nonexistent = f"nosuchuser_test42@{cmsetup.maildomain}"
+
+    msg = maildata(
+        "encrypted.eml",
+        from_addr=sender.addr,
+        to_addr=nonexistent,
+    ).as_string()
+    sender.smtp.sendmail(sender.addr, [nonexistent], msg)
+
+    def bounce_in_inbox():
+        messages = sender.imap.fetch_all_messages()
+        for m in messages:
+            if "mail delivery" in m.lower() or "undelivered" in m.lower():
+                return m
+        raise ValueError("bounce not yet in inbox")
+
+    bounce = try_n_times(30, bounce_in_inbox)
+    assert "nosuchuser_test42" in bounce
+
+
 def try_n_times(n, f):
     for _ in range(n - 1):
         try:

cmdeploy/src/cmdeploy/tests/plugin.py

@@ -19,6 +19,7 @@ def format_mail_domain(raw_domain: str) -> str:
     DomainValidator().validate_domain_re(raw_domain)
     return raw_domain
 
+
 conftestdir = Path(__file__).parent
 
 
@@ -466,6 +467,11 @@ def cmsetup(maildomain, gencreds, ssl_context):
     return CMSetup(maildomain, gencreds, ssl_context)
 
 
+@pytest.fixture
+def cmsetup2(maildomain2, gencreds, ssl_context):
+    return CMSetup(maildomain2, gencreds, ssl_context)
+
+
 class CMSetup:
     def __init__(self, maildomain, gencreds, ssl_context):
         self.maildomain = maildomain
@@ -476,7 +482,7 @@ class CMSetup:
         print(f"Creating {num} online users")
         users = []
         for i in range(num):
-            addr, password = self.gencreds()
+            addr, password = self.gencreds(format_mail_domain(self.maildomain))
             user = CMUser(self.maildomain, addr, password, self.ssl_context)
             assert user.smtp
             users.append(user)