diff --git a/federationapi/routing/send.go b/federationapi/routing/send.go
index 9c750923..892f09b0 100644
--- a/federationapi/routing/send.go
+++ b/federationapi/routing/send.go
@@ -106,8 +106,8 @@ func Send(
 		eduAPI:     eduAPI,
 		keys:       keys,
 		federation: federation,
+		hadEvents:  make(map[string]bool),
 		haveEvents: make(map[string]*gomatrixserverlib.HeaderedEvent),
-		newEvents:  make(map[string]bool),
 		keyAPI:     keyAPI,
 		roomsMu:    mu,
 	}
@@ -167,13 +167,12 @@ type txnReq struct {
 	servers      []gomatrixserverlib.ServerName
 	serversMutex sync.RWMutex
 	roomsMu      *internal.MutexByRoom
+	// a list of events from the auth and prev events which we already had
+	hadEvents map[string]bool
 	// local cache of events for auth checks, etc - this may include events
 	// which the roomserver is unaware of.
 	haveEvents map[string]*gomatrixserverlib.HeaderedEvent
-	// new events which the roomserver does not know about
-	newEvents      map[string]bool
-	newEventsMutex sync.RWMutex
-	work           string // metrics
+	work       string // metrics
 }
 
 // A subset of FederationClient functionality that txn requires. Useful for testing.
@@ -340,19 +339,6 @@ func (e missingPrevEventsError) Error() string {
 	return fmt.Sprintf("unable to get prev_events for event %q: %s", e.eventID, e.err)
 }
 
-func (t *txnReq) haveEventIDs() map[string]bool {
-	t.newEventsMutex.RLock()
-	defer t.newEventsMutex.RUnlock()
-	result := make(map[string]bool, len(t.haveEvents))
-	for eventID := range t.haveEvents {
-		if t.newEvents[eventID] {
-			continue
-		}
-		result[eventID] = true
-	}
-	return result
-}
-
 func (t *txnReq) processEDUs(ctx context.Context) {
 	for _, e := range t.EDUs {
 		eduCountTotal.Inc()
@@ -517,6 +503,15 @@ func (t *txnReq) processEvent(ctx context.Context, e *gomatrixserverlib.Event) e
 		return fmt.Errorf("t.rsAPI.QueryMissingAuthPrevEvents: %w", err)
 	}
 
+	// Prepare a map of all the events we already had before this point, so
+	// that we don't send them to the roomserver again.
+	for _, eventID := range append(e.AuthEventIDs(), e.PrevEventIDs()...) {
+		t.hadEvents[eventID] = true
+	}
+	for _, eventID := range append(stateResp.MissingAuthEventIDs, stateResp.MissingPrevEventIDs...) {
+		t.hadEvents[eventID] = false
+	}
+
 	if !stateResp.RoomExists {
 		// TODO: When synapse receives a message for a room it is not in it
 		// asks the remote server for the state of the room so that it can
@@ -739,7 +734,7 @@ func (t *txnReq) processEventWithMissingState(
 		api.KindOld,
 		resolvedState,
 		backwardsExtremity.Headered(roomVersion),
-		t.haveEventIDs(),
+		t.hadEvents,
 	)
 	if err != nil {
 		return fmt.Errorf("api.SendEventWithState: %w", err)
@@ -1235,9 +1230,5 @@ func (t *txnReq) lookupEvent(ctx context.Context, roomVersion gomatrixserverlib.
 		util.GetLogger(ctx).WithError(err).Warnf("Transaction: Couldn't validate signature of event %q", event.EventID())
 		return nil, verifySigError{event.EventID(), err}
 	}
-	h := t.cacheAndReturn(event.Headered(roomVersion))
-	t.newEventsMutex.Lock()
-	t.newEvents[h.EventID()] = true
-	t.newEventsMutex.Unlock()
-	return h, nil
+	return t.cacheAndReturn(event.Headered(roomVersion)), nil
 }