diff --git a/courier/smtp.go b/courier/smtp.go
index 3de84b00384c..3c60b3361594 100644
--- a/courier/smtp.go
+++ b/courier/smtp.go
@@ -218,7 +218,7 @@ func (c *courier) dispatchEmail(ctx context.Context, msg Message) error {
 		var mailErr *gomail.SendError
 
 		switch {
-		case errors.As(err, &mailErr) && mailErr.Index >= 500:
+		case errors.As(err, &mailErr) && errors.As(mailErr.Cause, &protoErr) && protoErr.Code >= 500:
 			fallthrough
 		case errors.As(err, &protoErr) && protoErr.Code >= 500:
 			// See https://en.wikipedia.org/wiki/List_of_SMTP_server_return_codes