diff --git a/federationsender/queue/destinationqueue.go b/federationsender/queue/destinationqueue.go
index 70cac5ed6..7dbffbf0d 100644
--- a/federationsender/queue/destinationqueue.go
+++ b/federationsender/queue/destinationqueue.go
@@ -246,6 +246,14 @@ func (oq *destinationQueue) backgroundSend() {
 			// has exceeded a maximum allowable value. Clean up the in-memory
 			// buffers at this point. The PDU clean-up is already on a defer.
 			log.Warnf("Blacklisting %q due to exceeding backoff threshold", oq.destination)
+			for i := pendingPDUs {
+				pendingPDUs[i] = nil
+			}
+			for i := pendingEDUs {
+				pendingEDUs[i] = nil
+			}
+			pendingPDUs = nil
+			pendingEDUs = nil
 			return
 		}
 		if until != nil && until.After(time.Now()) {
@@ -269,6 +277,12 @@ 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()
+			for i := pendingPDUs {
+				pendingPDUs[i] = nil
+			}
+			for i := pendingEDUs {
+				pendingEDUs[i] = nil
+			}
 			pendingPDUs = pendingPDUs[:0]
 			pendingEDUs = pendingEDUs[:0]
 		}