diff --git a/zerver/lib/actions.py b/zerver/lib/actions.py
index 2006779fd3..732d1c09ff 100644
--- a/zerver/lib/actions.py
+++ b/zerver/lib/actions.py
@@ -628,13 +628,14 @@ def do_send_message(message, rendered_content = None, no_log = False, stream = N
                               'stream': stream,
                               'local_id': local_id}])[0]
 
-def render_incoming_message(message, content):
-    # type: (Message, text_type) -> text_type
+def render_incoming_message(message, content, message_users):
+    # type: (Message, text_type, Set[UserProfile]) -> text_type
     realm_alert_words = alert_words_in_realm(message.get_realm())
     try:
         rendered_content = message.render_markdown(
             content=content,
             realm_alert_words=realm_alert_words,
+            message_users=message_users,
         )
     except BugdownRenderingException:
         raise JsonableError(_('Unable to render message'))
@@ -704,7 +705,8 @@ def do_send_messages(messages):
         assert message['message'].rendered_content is None
         rendered_content = render_incoming_message(
             message['message'],
-            message['message'].content)
+            message['message'].content,
+            message_users=message['active_recipients'])
         message['message'].set_rendered_content(rendered_content)
 
     for message in messages:
diff --git a/zerver/models.py b/zerver/models.py
index d7c0f328cd..17afb94ce3 100644
--- a/zerver/models.py
+++ b/zerver/models.py
@@ -809,8 +809,8 @@ class Message(ModelReprMixin, models.Model):
         # type: () -> Realm
         return self.sender.realm
 
-    def render_markdown(self, content, domain=None, realm_alert_words=None):
-        # type: (text_type, Optional[text_type], Optional[RealmAlertWords]) -> text_type
+    def render_markdown(self, content, domain=None, realm_alert_words=None, message_users=None):
+        # type: (text_type, Optional[text_type], Optional[RealmAlertWords], Set[UserProfile]) -> text_type
         """Return HTML for given markdown. Bugdown may add properties to the
         message object such as `mentions_user_ids` and `mentions_wildcard`.
         These are only on this Django object and are not saved in the
@@ -822,6 +822,11 @@ class Message(ModelReprMixin, models.Model):
             import zerver.lib.bugdown as bugdown
             # 'from zerver.lib import bugdown' gives mypy error in python 3 mode.
 
+        if message_users is None:
+            message_user_ids = set() # type: Set[int]
+        else:
+            message_user_ids = {u.id for u in message_users}
+
         self.mentions_wildcard = False
         self.is_me_message = False
         self.mentions_user_ids = set() # type: Set[int]
@@ -836,8 +841,9 @@ class Message(ModelReprMixin, models.Model):
 
         possible_words = set() # type: Set[text_type]
         if realm_alert_words is not None:
-            for words in realm_alert_words.values():
-                possible_words.update(set(words))
+            for user_id, words in realm_alert_words.items():
+                if user_id in message_user_ids:
+                    possible_words.update(set(words))
 
         # DO MAIN WORK HERE -- call bugdown to convert
         rendered_content = bugdown.convert(content, domain, self, possible_words)
@@ -846,8 +852,9 @@ class Message(ModelReprMixin, models.Model):
 
         if realm_alert_words is not None:
             for user_id, words in realm_alert_words.items():
-                if set(words).intersection(self.alert_words):
-                    self.user_ids_with_alert_words.add(user_id)
+                if user_id in message_user_ids:
+                    if set(words).intersection(self.alert_words):
+                        self.user_ids_with_alert_words.add(user_id)
 
         self.is_me_message = Message.is_status_message(content, rendered_content)
 
diff --git a/zerver/tests/test_bugdown.py b/zerver/tests/test_bugdown.py
index a3248133b9..ec191da3cc 100644
--- a/zerver/tests/test_bugdown.py
+++ b/zerver/tests/test_bugdown.py
@@ -421,7 +421,9 @@ class BugdownTest(TestCase):
         realm_alert_words = alert_words_in_realm(user_profile.realm)
 
         def render(msg, content):
-            return msg.render_markdown(content, realm_alert_words=realm_alert_words)
+            return msg.render_markdown(content,
+                                       realm_alert_words=realm_alert_words,
+                                       message_users={user_profile})
 
         content = "We have an ALERTWORD day today!"
         self.assertEqual(render(msg, content), "<p>We have an ALERTWORD day today!</p>")
diff --git a/zerver/views/messages.py b/zerver/views/messages.py
index f9f65a5af5..46aafb7df9 100644
--- a/zerver/views/messages.py
+++ b/zerver/views/messages.py
@@ -33,7 +33,7 @@ from zerver.models import Message, UserProfile, Stream, Subscription, \
     parse_usermessage_flags, to_dict_cache_key_id, extract_message_dict, \
     stringify_message_dict, \
     resolve_email_to_domain, get_realm, get_active_streams, \
-    bulk_get_streams
+    bulk_get_streams, get_user_profile_by_id
 
 from sqlalchemy import func
 from sqlalchemy.sql import select, join, column, literal_column, literal, and_, \
@@ -911,8 +911,18 @@ def update_message_backend(request, user_profile,
             raise JsonableError(_("Content can't be empty"))
         content = truncate_body(content)
 
+        # We exclude UserMessage.flags.historical rows since those
+        # users did not receive the message originally, and thus
+        # probably are not relevant for reprocessed alert_words,
+        # mentions and similar rendering features.  This may be a
+        # decision we change in the future.
+        ums = UserMessage.objects.filter(message=message.id,
+                                         flags=~UserMessage.flags.historical)
+        message_users = {get_user_profile_by_id(um.user_profile_id) for um in ums}
         # If rendering fails, the called code will raise a JsonableError.
-        rendered_content = render_incoming_message(message, content)
+        rendered_content = render_incoming_message(message,
+                                                   content=content,
+                                                   message_users=message_users)
 
     do_update_message(user_profile, message, subject, propagate_mode, content, rendered_content)
     return json_success()