diff --git a/client/clientBean.go b/client/clientBean.go
index 014d2a509fd..d2f919a8639 100644
--- a/client/clientBean.go
+++ b/client/clientBean.go
@@ -191,7 +191,7 @@ func (h *clientBeanImpl) GetRemoteAdminClient(cluster string) (adminservice.Admi
 	if !ok {
 		clusterInfo, clusterFound := h.clusterMetadata.GetAllClusterInfo()[cluster]
 		if !clusterFound {
-			return nil, &serviceerror.Unavailable{
+			return nil, &serviceerror.NotFound{
 				Message: fmt.Sprintf(
 					"Unknown cluster name: %v with given cluster information map: %v.",
 					cluster,
@@ -233,7 +233,7 @@ func (h *clientBeanImpl) GetRemoteFrontendClient(cluster string) (workflowservic
 	if !ok {
 		clusterInfo, clusterFound := h.clusterMetadata.GetAllClusterInfo()[cluster]
 		if !clusterFound {
-			return nil, &serviceerror.Unavailable{
+			return nil, &serviceerror.NotFound{
 				Message: fmt.Sprintf(
 					"Unknown cluster name: %v with given cluster information map: %v.",
 					cluster,
diff --git a/service/history/replication/task_executor.go b/service/history/replication/task_executor.go
index 87d7b3577e6..376744a33dc 100644
--- a/service/history/replication/task_executor.go
+++ b/service/history/replication/task_executor.go
@@ -326,6 +326,10 @@ func (e *taskExecutorImpl) filterTask(
 
 	namespaceEntry, err := e.namespaceRegistry.GetNamespaceByID(namespaceID)
 	if err != nil {
+		if _, ok := err.(*serviceerror.NamespaceNotFound); ok {
+			// Drop the task
+			return false, nil
+		}
 		return false, err
 	}
 
diff --git a/service/history/replication/task_executor_test.go b/service/history/replication/task_executor_test.go
index 76adbc4930d..cbffe6b98dd 100644
--- a/service/history/replication/task_executor_test.go
+++ b/service/history/replication/task_executor_test.go
@@ -34,6 +34,7 @@ import (
 	"github.com/stretchr/testify/require"
 	"github.com/stretchr/testify/suite"
 	commonpb "go.temporal.io/api/common/v1"
+	"go.temporal.io/api/serviceerror"
 
 	enumsspb "go.temporal.io/server/api/enums/v1"
 	historyspb "go.temporal.io/server/api/history/v1"
@@ -181,6 +182,16 @@ func (s *taskExecutorSuite) TestFilterTask_EnforceApply() {
 	s.True(ok)
 }
 
+func (s *taskExecutorSuite) TestFilterTask_NamespaceNotFound() {
+	namespaceID := namespace.ID(uuid.New())
+	s.mockNamespaceCache.EXPECT().
+		GetNamespaceByID(namespaceID).
+		Return(nil, &serviceerror.NamespaceNotFound{})
+	ok, err := s.replicationTaskExecutor.filterTask(namespaceID, false)
+	s.NoError(err)
+	s.False(ok)
+}
+
 func (s *taskExecutorSuite) TestProcessTaskOnce_SyncActivityReplicationTask() {
 	namespaceID := namespace.ID(uuid.New())
 	workflowID := uuid.New()