diff --git a/src/github.com/matrix-org/dendrite/federationapi/writers/send.go b/src/github.com/matrix-org/dendrite/federationapi/writers/send.go
index 2b0a888f..006fc846 100644
--- a/src/github.com/matrix-org/dendrite/federationapi/writers/send.go
+++ b/src/github.com/matrix-org/dendrite/federationapi/writers/send.go
@@ -2,6 +2,7 @@ package writers
 
 import (
 	"encoding/json"
+	"fmt"
 	"github.com/matrix-org/dendrite/clientapi/jsonerror"
 	"github.com/matrix-org/dendrite/federationapi/config"
 	"github.com/matrix-org/dendrite/roomserver/api"
@@ -48,6 +49,8 @@ func processTransaction(t gomatrixserverlib.Transaction, query api.RoomserverQue
 
 }
 
+var errUnknownRoom = fmt.Errorf("unknown room")
+
 func processEvent(e gomatrixserverlib.Event, query api.RoomserverQueryAPI) error {
 	refs := e.PrevEvents()
 	prevEventIDs := make([]string, len(refs))
@@ -55,19 +58,57 @@ func processEvent(e gomatrixserverlib.Event, query api.RoomserverQueryAPI) error
 		prevEventIDs[i] = refs[i].EventID
 	}
 
+	// Fetch the state needed to authenticate the event.
 	needed := gomatrixserverlib.StateNeededForAuth([]gomatrixserverlib.Event{e})
-
-	request := api.QueryStateAfterEventsRequest{
+	stateReq := api.QueryStateAfterEventsRequest{
 		RoomID:       e.RoomID(),
 		PrevEventIDs: prevEventIDs,
 		StateToFetch: needed.Tuples(),
 	}
-	var response api.QueryStateAfterEventsResponse
-	if err := query.QueryStateAfterEvents(&request, &response); err != nil {
+	var stateResp api.QueryStateAfterEventsResponse
+	if err := query.QueryStateAfterEvents(&stateReq, &stateResp); err != nil {
 		return err
 	}
 
-	// TODO process the event.
+	if !stateResp.RoomExists {
+		// TODO: When synapse receives a message for a room it is not in it
+		// asked the remote server for the state of the room so that it can
+		// check if the remote server knows of a join "m.room.member" event
+		// that this server is unaware of.
+		// However generally speaking we should reject events for rooms we
+		// aren't a member of.
+		return errUnknownRoom
+	}
+
+	if !stateResp.PrevEventsExist {
+		// We are missing the previous events for this events.
+		// This means that there is a gap in our view of the history of the
+		// room. There two ways that we can handle such a gap:
+		//   1) We can fill in the gap using /get_missing_events
+		//   2) We can leave the gap and request the state of the room at
+		//      this event from the remote server using either /state_ids
+		//      or /state.
+		// Synapse will attempt to do 1 and if that fails or if the gap is
+		// too large then it will attempt 2.
+		// Synapse will use /state_ids if possible since ususally the state
+		// is largely unchanged and it is more efficient to fetch a list of
+		// event ids and then use /event to fetch the individual events.
+		// However not all version of synapse support /state_ids so you may
+		// need to fallback to /state.
+		// TODO: Attempt to fill in the gap using /get_missing_events
+		// TODO: Attempt to fetch the state using /state_ids and /events
+		// TODO: Attempt to fetch the state using /state
+		panic(fmt.Errorf("Receiving events with missing prev_events is no implemented"))
+	}
+
+	authUsingState := gomatrixserverlib.NewAuthEvents(nil)
+	for i := range stateResp.StateEvents {
+		authUsingState.AddEvent(&stateResp.StateEvents[i])
+	}
+	err := gomatrixserverlib.Allowed(e, &authUsingState)
+	if err != nil {
+		return err
+	}
 
 	return nil
 }