email_mirror: Fix exception handling unstructured headers.

This commit rewrites the way addresses are collected. If
the header with the address is not an AddressHeader (for instance,
Delivered-To and Envelope-To), we take its string representation.

Fixes: #15864 ("Error in email_mirror - _UnstructuredHeader has no attribute addresses").

zerver/lib/email_mirror.py

@@ -1,5 +1,6 @@
 import logging
 import re
+from email.headerregistry import AddressHeader
 from email.message import EmailMessage
 from typing import Dict, List, Optional, Tuple
 
@@ -310,9 +311,14 @@ def find_emailgateway_recipient(message: EmailMessage) -> str:
 
     for header_name in recipient_headers:
         for header_value in message.get_all(header_name, []):
-            for addr in header_value.addresses:
-                if match_email_re.match(addr.addr_spec):
-                    return addr.addr_spec
+            if isinstance(header_value, AddressHeader):
+                emails = [addr.addr_spec for addr in header_value.addresses]
+            else:
+                emails = [str(header_value)]
+
+            for email in emails:
+                if match_email_re.match(email):
+                    return email
 
     raise ZulipEmailForwardError("Missing recipient in mirror email")
 

zerver/tests/test_email_mirror.py

@@ -232,6 +232,36 @@ class TestStreamEmailMessagesSuccess(ZulipTestCase):
         self.assertEqual(get_display_recipient(message.recipient), stream.name)
         self.assertEqual(message.topic_name(), incoming_valid_message['Subject'])
 
+    # Test receiving an email with the address on an UnstructuredHeader
+    # (e.g. Envelope-To) instead of an AddressHeader (e.g. To).
+    # https://github.com/zulip/zulip/issues/15864
+    def test_receive_stream_email_messages_other_header_success(self) -> None:
+        user_profile = self.example_user('hamlet')
+        self.login_user(user_profile)
+        self.subscribe(user_profile, "Denmark")
+        stream = get_stream("Denmark", user_profile.realm)
+
+        stream_to_address = encode_email_address(stream)
+
+        incoming_valid_message = EmailMessage()
+        incoming_valid_message.set_content('TestStreamEmailMessages Body')
+
+        incoming_valid_message['Subject'] = 'TestStreamEmailMessages Subject'
+        incoming_valid_message['From'] = self.example_email('hamlet')
+        # Simulate a mailing list
+        incoming_valid_message['To'] = "foo-mailinglist@example.com"
+        incoming_valid_message['Envelope-To'] = stream_to_address
+        incoming_valid_message['Reply-to'] = self.example_email('othello')
+
+        process_message(incoming_valid_message)
+
+        # Hamlet is subscribed to this stream so should see the email message from Othello.
+        message = most_recent_message(user_profile)
+
+        self.assertEqual(message.content, "TestStreamEmailMessages Body")
+        self.assertEqual(get_display_recipient(message.recipient), stream.name)
+        self.assertEqual(message.topic_name(), incoming_valid_message['Subject'])
+
     def test_receive_stream_email_messages_blank_subject_success(self) -> None:
         user_profile = self.example_user('hamlet')
         self.login_user(user_profile)