email: Don't send overly-long "To" addresses.

This parallels b7fa41601d, but with "To"
addresses, not "From" addresses.

zerver/lib/send_email.py

@@ -86,10 +86,20 @@ def build_email(
         if realm is None:
             assert len({to_user.realm_id for to_user in to_users}) == 1
             realm = to_users[0].realm
-        to_emails = [
-            str(Address(display_name=to_user.full_name, addr_spec=to_user.delivery_email))
-            for to_user in to_users
-        ]
+        to_emails = []
+        for to_user in to_users:
+            stringified = str(
+                Address(display_name=to_user.full_name, addr_spec=to_user.delivery_email)
+            )
+            # Check ASCII encoding length.  Amazon SES rejects emails
+            # with From or To values longer than 320 characters (which
+            # appears to be a misinterpretation of the RFC); in that
+            # case we drop the name part from the address, under the
+            # theory that it's better to send the email with a
+            # simplified field than not at all.
+            if len(sanitize_address(stringified, "utf-8")) > 320:
+                stringified = str(Address(addr_spec=to_user.delivery_email))
+            to_emails.append(stringified)
 
     extra_headers = {}
     if realm is not None:
@@ -159,11 +169,8 @@ def build_email(
 
     # Set the "From" that is displayed separately from the envelope-from.
     extra_headers["From"] = str(Address(display_name=from_name, addr_spec=from_address))
-    # Check ASCII encoding length.  Amazon SES rejects emails with
-    # From names longer than 320 characters (which appears to be a
-    # misinterpretation of the RFC); in that case we drop the name
-    # from the From line, under the theory that it's better to send
-    # the email with a simplified From field than not.
+    # As above, with the "To" line, we drop the name part if it would
+    # result in an address which is longer than 320 bytes.
     if len(sanitize_address(extra_headers["From"], "utf-8")) > 320:
         extra_headers["From"] = str(Address(addr_spec=from_address))
 

zerver/tests/test_send_email.py

@@ -40,6 +40,34 @@ class TestBuildEmail(ZulipTestCase):
         )
         self.assertEqual(mail.extra_headers["From"], FromAddress.NOREPLY)
 
+    def test_limited_to_length(self) -> None:
+        hamlet = self.example_user("hamlet")
+        # This is exactly the max length
+        limit_length_name = "澳" * 61
+        hamlet.full_name = limit_length_name
+        hamlet.save()
+
+        mail = build_email(
+            "zerver/emails/password_reset",
+            to_user_ids=[hamlet.id],
+            from_name="Noreply",
+            from_address=FromAddress.NOREPLY,
+            language="en",
+        )
+        self.assertEqual(mail.to[0], f"{hamlet.full_name} <{hamlet.delivery_email}>")
+
+        # One more character makes it flip to just the address, with no name
+        hamlet.full_name += "澳"
+        hamlet.save()
+        mail = build_email(
+            "zerver/emails/password_reset",
+            to_user_ids=[hamlet.id],
+            from_name="Noreply",
+            from_address=FromAddress.NOREPLY,
+            language="en",
+        )
+        self.assertEqual(mail.to[0], hamlet.delivery_email)
+
 
 class TestSendEmail(ZulipTestCase):
     def test_initialize_connection(self) -> None: