Issue #3375366: Don't automatically generate plain text version

src/Plugin/Mail/SymfonyMailer.php

@@ -151,7 +151,7 @@ class SymfonyMailer implements MailInterface, ContainerFactoryPluginInterface {
     // Determine if a plain text alternative is required. The message parameter
     // takes priority over config. Support the alternate parameter 'convert'
     // for back-compatibility.
-    $generate_plain = TRUE; //$message['params']['generate_plain'] ?? $message['params']['convert'] ?? $this->config['message']['generate_plain'];
+    $generate_plain = !empty($message['params']['generate_plain']) || !empty($message['params']['convert']);
 
     if ($generate_plain && empty($message['plain']) && $is_html) {
       // Generate plain text alternative. This must be done first with the

symfony_mailer_lite.module

@@ -65,5 +65,6 @@ function symfony_mailer_lite_mail($key, &$message) {
     $message['body'] = array_map(function ($text) {
       return \Drupal\Core\Render\Markup::create($text);
     }, $text);
+    $message['params']['generate_plain'] = TRUE;
   }
 }