diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index c512d582c8..f0f4398d45 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -799,8 +799,10 @@ class Auth(object):
         if self.hs.config.limit_usage_by_mau is True:
             # If the user is already part of the MAU cohort
             if user_id:
-                activity = yield self.store.user_last_seen_monthly_active(user_id)
-                if activity:
+                activity, is_trial = yield self.store.user_last_seen_monthly_active(
+                    user_id,
+                )
+                if activity or is_trial:
                     return
             # Else if there is no room in the MAU bucket, bail
             current_mau = yield self.store.get_monthly_active_count()
diff --git a/synapse/server_notices/resource_limits_server_notices.py b/synapse/server_notices/resource_limits_server_notices.py
index 96eb97771f..04ff15fbd3 100644
--- a/synapse/server_notices/resource_limits_server_notices.py
+++ b/synapse/server_notices/resource_limits_server_notices.py
@@ -66,8 +66,8 @@ class ResourceLimitsServerNotices(object):
         if self._config.limit_usage_by_mau is False:
             return
 
-        timestamp = yield self._store.user_last_seen_monthly_active(user_id)
-        if timestamp is None:
+        timestamp, is_trial = yield self._store.user_last_seen_monthly_active(user_id)
+        if timestamp is None or is_trial:
             # This user will be blocked from receiving the notice anyway.
             # In practice, not sure we can ever get here
             return
diff --git a/synapse/storage/monthly_active_users.py b/synapse/storage/monthly_active_users.py
index 8169f77b1e..129705bd39 100644
--- a/synapse/storage/monthly_active_users.py
+++ b/synapse/storage/monthly_active_users.py
@@ -128,7 +128,7 @@ class MonthlyActiveUsersStore(SQLBaseStore):
         # is racy.
         # Have resolved to invalidate the whole cache for now and do
         # something about it if and when the perf becomes significant
-        self.user_last_seen_monthly_active.invalidate_all()
+        self._user_last_seen_monthly_active.invalidate_all()
         self.get_monthly_active_count.invalidate_all()
 
     @cached(num_args=0)
@@ -168,41 +168,54 @@ class MonthlyActiveUsersStore(SQLBaseStore):
             lock=False,
         )
         if is_insert:
-            self.user_last_seen_monthly_active.invalidate((user_id,))
+            self._user_last_seen_monthly_active.invalidate((user_id,))
             self.get_monthly_active_count.invalidate(())
 
-    @cached(num_args=1)
+    @defer.inlineCallbacks
     def user_last_seen_monthly_active(self, user_id):
+        """Checks if a given user is part of the monthly active user group
+
+        Args:
+            user_id (str): user to add/update
+
+        Return:
+            Deferred[(int|None, bool)]: First arg is None if the user is not
+                in the mau group, otherwise approximate timestamp they were last
+                seen in milliseconds.
+                The second is a bool indicating if the user is in the trial
+                period or not.
         """
-            Checks if a given user is part of the monthly active user group
-            Arguments:
-                user_id (str): user to add/update
-            Return:
-                Deferred[(int,bool)|None] : (time since last seen, bool if trial user)
-                    None if never seen
-        """
+
+        ret = yield self._user_last_seen_monthly_active(user_id)
+        last_seen, created_at = ret
 
         mau_trial_ms = self.hs.config.mau_trial_days * 24 * 60 * 60 * 1000
+        is_trial = (self._clock.time_msec() - created_at) < mau_trial_ms
+
+        defer.returnValue((last_seen, is_trial))
+
+    @cached(num_args=1)
+    def _user_last_seen_monthly_active(self, user_id):
+        """Checks if a given user is part of the monthly active user group
+
+        Args:
+            user_id (str): user to add/update
+
+        Return:
+            Deferred[(int, int)|None]: None if never seen, otherwise time user
+            was last seen and registration time of user (both in milliseconds)
+        """
 
         def _user_last_seen_monthly_active(txn):
-
             sql = """
                 SELECT timestamp, creation_ts
-                FROM monthly_active_users LEFT JOIN users
+                FROM users LEFT JOIN monthly_active_users
                 ON monthly_active_users.user_id = users.name
-                WHERE user_id = ?
+                WHERE name = ?
             """
-
             txn.execute(sql, (user_id,))
-            rows = txn.fetchall()
-            if not rows or not rows[0][0]:
-                return None
-            else:
-                (timestamp, created) = (rows[0][0], rows[0][1])
-                if not created:
-                    created = 0
-                trial = (timestamp - created * 1000) < mau_trial_ms
-                return (timestamp, trial)
+            row = txn.fetchone()
+            return row[0], row[1] * 1000
 
         return self.runInteraction(
             "user_last_seen_monthly_active",
@@ -218,9 +231,9 @@ class MonthlyActiveUsersStore(SQLBaseStore):
             user_id(str): the user_id to query
         """
         if self.hs.config.limit_usage_by_mau:
-            activity = yield self.user_last_seen_monthly_active(user_id)
+            last_seen, is_trial = yield self.user_last_seen_monthly_active(user_id)
 
-            if activity and activity[1]:
+            if is_trial:
                 # we don't track trial users in the MAU table.
                 return
 
@@ -231,9 +244,9 @@ class MonthlyActiveUsersStore(SQLBaseStore):
             # We always insert new users (where MAU threshold has not been reached),
             # but only update if we have not previously seen the user for
             # LAST_SEEN_GRANULARITY ms
-            if activity is None:
+            if last_seen is None:
                 count = yield self.get_monthly_active_count()
                 if count < self.hs.config.max_mau_value:
                     yield self.upsert_monthly_active_user(user_id)
-            elif now - activity[0] > LAST_SEEN_GRANULARITY:
+            elif now - last_seen > LAST_SEEN_GRANULARITY:
                 yield self.upsert_monthly_active_user(user_id)