diff --git a/src/Plugin/Mail/SymfonyMailer.php b/src/Plugin/Mail/SymfonyMailer.php
index 2b44c043ab4940d5020889456cbb7e8568f92543..db01470c6722b71f86538e840a065abb3761930e 100644
--- a/src/Plugin/Mail/SymfonyMailer.php
+++ b/src/Plugin/Mail/SymfonyMailer.php
@@ -713,7 +713,7 @@ class SymfonyMailer implements MailInterface, ContainerFactoryPluginInterface {
   protected function getContentType(array $message) : string {
     // The message parameter takes priority over config. Support the alternate
     // parameter 'format' for back-compatibility.
-    $content_type = $message['params']['content_type'] ?? $message['params']['format'] ?? 'text/html';
+    $content_type = $message['params']['content_type'] ?? $message['params']['format'] ?? NULL;
     // 1) check the message parameters.
     if ($content_type) {
       return $content_type;