diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 59d0ad3bdc..88bd2d572e 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -279,6 +279,8 @@ class MessageHandler(BaseHandler):
 
         if event.type == EventTypes.Message:
             presence = self.hs.get_presence_handler()
+            # We don't want to block sending messages on any presence code. This
+            # matters as sometimes presence code can take a while.
             preserve_fn(presence.bump_presence_active_time)(user)
 
     @defer.inlineCallbacks