diff --git a/federationsender/api/api.go b/federationsender/api/api.go
index 281165e84..cea0010d6 100644
--- a/federationsender/api/api.go
+++ b/federationsender/api/api.go
@@ -27,7 +27,7 @@ type FederationClientError struct {
 }
 
 func (e *FederationClientError) Error() string {
-	return fmt.Sprintf("%s - (retry_after=%d, blacklisted=%v", e.Err, e.RetryAfter, e.Blacklisted)
+	return fmt.Sprintf("%s - (retry_after=%d, blacklisted=%v)", e.Err, e.RetryAfter, e.Blacklisted)
 }
 
 // FederationSenderInternalAPI is used to query information from the federation sender.
diff --git a/federationsender/internal/api.go b/federationsender/internal/api.go
index 956981377..dd7cb63cf 100644
--- a/federationsender/internal/api.go
+++ b/federationsender/internal/api.go
@@ -10,6 +10,7 @@ import (
 	"github.com/matrix-org/dendrite/federationsender/storage"
 	"github.com/matrix-org/dendrite/internal/config"
 	roomserverAPI "github.com/matrix-org/dendrite/roomserver/api"
+	"github.com/matrix-org/gomatrix"
 	"github.com/matrix-org/gomatrixserverlib"
 	"go.uber.org/atomic"
 )
@@ -67,6 +68,21 @@ func (a *FederationSenderInternalAPI) isBlacklistedOrBackingOff(s gomatrixserver
 	return stats, nil
 }
 
+func failBlacklistableError(err error, stats *statistics.ServerStatistics) {
+	if err == nil {
+		return
+	}
+	mxerr, ok := err.(gomatrix.HTTPError)
+	if !ok {
+		stats.Failure()
+		return
+	}
+	if mxerr.Code == 500 {
+		stats.Failure()
+		return
+	}
+}
+
 func (a *FederationSenderInternalAPI) GetUserDevices(
 	ctx context.Context, s gomatrixserverlib.ServerName, userID string,
 ) (gomatrixserverlib.RespUserDevices, error) {
@@ -77,7 +93,7 @@ func (a *FederationSenderInternalAPI) GetUserDevices(
 	}
 	res, err = a.federation.GetUserDevices(ctx, s, userID)
 	if err != nil {
-		stats.Failure()
+		failBlacklistableError(err, stats)
 		return res, &api.FederationClientError{
 			Err: err.Error(),
 		}
@@ -96,7 +112,7 @@ func (a *FederationSenderInternalAPI) ClaimKeys(
 	}
 	res, err = a.federation.ClaimKeys(ctx, s, oneTimeKeys)
 	if err != nil {
-		stats.Failure()
+		failBlacklistableError(err, stats)
 		return res, &api.FederationClientError{
 			Err: err.Error(),
 		}
@@ -115,7 +131,7 @@ func (a *FederationSenderInternalAPI) QueryKeys(
 	}
 	res, err = a.federation.QueryKeys(ctx, s, keys)
 	if err != nil {
-		stats.Failure()
+		failBlacklistableError(err, stats)
 		return res, &api.FederationClientError{
 			Err: err.Error(),
 		}
diff --git a/federationsender/inthttp/client.go b/federationsender/inthttp/client.go
index 846c6d746..79e220c38 100644
--- a/federationsender/inthttp/client.go
+++ b/federationsender/inthttp/client.go
@@ -159,7 +159,7 @@ func (h *httpFederationSenderInternalAPI) GetUserDevices(
 	}
 	var response getUserDevices
 	apiURL := h.federationSenderURL + FederationSenderGetUserDevicesPath
-	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, &request, &response)
 	if err != nil {
 		return result, err
 	}
@@ -189,7 +189,7 @@ func (h *httpFederationSenderInternalAPI) ClaimKeys(
 	}
 	var response claimKeys
 	apiURL := h.federationSenderURL + FederationSenderClaimKeysPath
-	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, &request, &response)
 	if err != nil {
 		return result, err
 	}
@@ -219,7 +219,7 @@ func (h *httpFederationSenderInternalAPI) QueryKeys(
 	}
 	var response queryKeys
 	apiURL := h.federationSenderURL + FederationSenderQueryKeysPath
-	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, request, response)
+	err := httputil.PostJSON(ctx, span, h.httpClient, apiURL, &request, &response)
 	if err != nil {
 		return result, err
 	}