Prevent missed-message emails for deleted messages.

This of course only works in the 2 minute window where missed-message
emails are planned, but nonetheless likely avoids common cases of
emailing users with deleted messages.

Fixes: #3873.

zerver/lib/notifications.py

@@ -335,6 +335,10 @@ def handle_missedmessage_emails(user_profile_id, missed_email_events):
     messages = Message.objects.filter(usermessage__user_profile_id=user_profile,
                                       id__in=message_ids,
                                       usermessage__flags=~UserMessage.flags.read)
+
+    # Cancel missed-message emails for deleted messages
+    messages = [um for um in messages if um.content != "(deleted)"]
+
     if not messages:
         return