diff --git a/federationsender/queue/destinationqueue.go b/federationsender/queue/destinationqueue.go
index 9f3be8875..5f1c4ddec 100644
--- a/federationsender/queue/destinationqueue.go
+++ b/federationsender/queue/destinationqueue.go
@@ -51,7 +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
+	pendingPDUs      atomic.Int32                            // how many PDUs are waiting to be sent
 	pendingEDUs      []*gomatrixserverlib.EDU                // owned by backgroundSend
 	pendingInvites   []*gomatrixserverlib.InviteV2Request    // owned by backgroundSend
 	retryServerCh    chan bool                               // interrupts backoff
@@ -119,9 +119,8 @@ 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.
-	if oq.pendingPDUs.CAS(false, true) {
-		oq.incomingPDUs <- struct{}{}
-	}
+	oq.pendingPDUs.Add(1)
+	oq.incomingPDUs <- struct{}{}
 }
 
 // sendEDU adds the EDU event to the pending queue for the destination.
@@ -224,7 +223,7 @@ func (oq *destinationQueue) backgroundSend() {
 		}
 
 		// If we have pending PDUs or EDUs then construct a transaction.
-		if oq.pendingPDUs.Load() || len(oq.pendingEDUs) > 0 {
+		if oq.pendingPDUs.Load() > 0 || 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
@@ -250,7 +249,6 @@ 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()
@@ -357,8 +355,9 @@ func (oq *destinationQueue) nextTransaction(
 	switch e := err.(type) {
 	case nil:
 		// No error was returned so the transaction looks to have
-		// been successfully sent. Clean up the transaction in the
-		// database.
+		// been successfully sent.
+		oq.pendingPDUs.Sub(int32(len(t.PDUs)))
+		// Clean up the transaction in the database.
 		if err = oq.db.CleanTransactionPDUs(
 			context.TODO(),
 			t.Destination,