diff --git a/synapse/storage/deviceinbox.py b/synapse/storage/deviceinbox.py
index d9f91ccc4e..61da0e89e6 100644
--- a/synapse/storage/deviceinbox.py
+++ b/synapse/storage/deviceinbox.py
@@ -42,7 +42,15 @@ class DeviceInboxStore(SQLBaseStore):
             inserted.
         """
 
-        def add_messages_to_device_federation_outbox(txn, now_ms, stream_id):
+        def add_messages_txn(txn, now_ms, stream_id):
+            # Add the local messages directly to the local inbox.
+            self._add_messages_to_local_device_inbox_txn(
+                txn, stream_id, local_messages_by_user_then_device
+            )
+
+            # Add the remote messages to the federation outbox.
+            # We'll send them to a remote server when we next send a
+            # federation transaction to that destination.
             sql = (
                 "INSERT INTO device_federation_outbox"
                 " (destination, stream_id, queued_ts, messages_json)"
@@ -52,15 +60,8 @@ class DeviceInboxStore(SQLBaseStore):
             for destination, edu in remote_messages_by_destination.items():
                 edu_json = ujson.dumps(edu)
                 rows.append((destination, stream_id, now_ms, edu_json))
-
             txn.executemany(sql, rows)
 
-        def add_messages_txn(txn, now_ms, stream_id):
-            self._add_messages_to_local_device_inbox_txn(
-                txn, stream_id, local_messages_by_user_then_device
-            )
-            add_messages_to_device_federation_outbox(txn, now_ms, stream_id)
-
         with self._device_inbox_id_gen.get_next() as stream_id:
             now_ms = self.clock.time_msec()
             yield self.runInteraction(
@@ -77,6 +78,9 @@ class DeviceInboxStore(SQLBaseStore):
         self, origin, message_id, local_messages_by_user_then_device
     ):
         def add_messages_txn(txn, now_ms, stream_id):
+            # Check if we've already inserted a matching message_id for that
+            # origin. This can happen if the origin doesn't receive our
+            # acknowledgement from the first time we received the message.
             already_inserted = self._simple_select_one_txn(
                 txn, table="device_federation_inbox",
                 keyvalues={"origin": origin, "message_id": message_id},
@@ -86,6 +90,8 @@ class DeviceInboxStore(SQLBaseStore):
             if already_inserted is not None:
                 return
 
+            # Add an entry for this message_id so that we know we've processed
+            # it.
             self._simple_insert_txn(
                 txn, table="device_federation_inbox",
                 values={
@@ -95,6 +101,8 @@ class DeviceInboxStore(SQLBaseStore):
                 },
             )
 
+            # Add the messages to the approriate local device inboxes so that
+            # they'll be sent to the devices when they next sync.
             self._add_messages_to_local_device_inbox_txn(
                 txn, stream_id, local_messages_by_user_then_device
             )