diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index bd4f847..a363ea0 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -18,9 +18,9 @@ jobs:
           coverage: xdebug
       - name: Get composer cache directory
         id: composer-cache
-        run: echo "::set-output name=dir::$(composer config cache-files-dir)"
+        run: echo "dir=$(composer config cache-files-dir)" >> $GITHUB_OUTPUT
       - name: Cache composer dependencies
-        uses: actions/cache@v1
+        uses: actions/cache@v4
         with:
           path: ${{ steps.composer-cache.outputs.dir }}
           key: ${{ runner.os }}-composer-${{ hashFiles('**/composer.json') }}
diff --git a/src/CssInlinerPlugin.php b/src/CssInlinerPlugin.php
index 5f634a5..22e2cff 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(