diff --git a/federationsender/queue/destinationqueue.go b/federationsender/queue/destinationqueue.go
index 0d62b4361..9f3be8875 100644
--- a/federationsender/queue/destinationqueue.go
+++ b/federationsender/queue/destinationqueue.go
@@ -51,6 +51,7 @@ type destinationQueue struct {
 	incomingEDUs     chan *gomatrixserverlib.EDU             // EDUs to send
 	transactionID    gomatrixserverlib.TransactionID         // last transaction ID
 	transactionCount int                                     // how many events in this transaction so far
+	pendingPDUs      atomic.Bool                             // there are PDUs waiting to be sent
 	pendingEDUs      []*gomatrixserverlib.EDU                // owned by backgroundSend
 	pendingInvites   []*gomatrixserverlib.InviteV2Request    // owned by backgroundSend
 	retryServerCh    chan bool                               // interrupts backoff
@@ -118,7 +119,9 @@ func (oq *destinationQueue) sendEvent(nid int64) {
 	}
 	// Signal that we've sent a new PDU. This will cause the queue to
 	// wake up if it's asleep.
-	oq.incomingPDUs <- struct{}{}
+	if oq.pendingPDUs.CAS(false, true) {
+		oq.incomingPDUs <- struct{}{}
+	}
 }
 
 // sendEDU adds the EDU event to the pending queue for the destination.
@@ -160,8 +163,6 @@ func (oq *destinationQueue) backgroundSend() {
 	defer oq.running.Store(false)
 
 	for {
-		pendingPDUs := false
-
 		// For now we don't know the next transaction ID. Set it to an
 		// empty one. The next step will populate it if we have pending
 		// PDUs in the database. Otherwise we'll generate one later on,
@@ -172,8 +173,8 @@ func (oq *destinationQueue) backgroundSend() {
 		// idle timeout.
 		select {
 		case <-oq.incomingPDUs:
-			// There are new PDUs waiting in the database.
-			pendingPDUs = true
+			// We were woken up because there are new PDUs waiting in the
+			// database.
 		case edu := <-oq.incomingEDUs:
 			// EDUs are handled in-memory for now. We will try to keep
 			// the ordering intact.
@@ -223,7 +224,7 @@ func (oq *destinationQueue) backgroundSend() {
 		}
 
 		// If we have pending PDUs or EDUs then construct a transaction.
-		if pendingPDUs || len(oq.pendingEDUs) > 0 {
+		if oq.pendingPDUs.Load() || len(oq.pendingEDUs) > 0 {
 			// If we haven't got a transaction ID then we should generate
 			// one. Ideally we'd know this already because something queued
 			// in the database would give us one, but if we're dealing with
@@ -249,6 +250,7 @@ func (oq *destinationQueue) backgroundSend() {
 				// If we successfully sent the transaction then clear out
 				// the pending events and EDUs, and wipe our transaction ID.
 				oq.statistics.Success()
+				oq.pendingPDUs.Store(false)
 				oq.transactionID = ""
 				// Clean up the in-memory buffers.
 				oq.cleanPendingEDUs()