diff --git a/src/CssInlinerPlugin.php b/src/CssInlinerPlugin.php
index 367028c..ee2ff17 100644
--- a/src/CssInlinerPlugin.php
+++ b/src/CssInlinerPlugin.php
@@ -96,7 +96,7 @@ private function handleSymfonyEmail(Email $message): void
 
         if ($body instanceof TextPart) {
             $message->setBody($this->processPart($body));
-        } elseif ($body instanceof AlternativePart || $body instanceof MixedPart) {
+        } elseif ($body instanceof AbstractMultipartPart) {
             $part_type = get_class($body);
             $message->setBody(new $part_type(
                 ...array_map(