Sends emails and retries failures not working for custom module emails

Thank you for the report. This is probably related to 🐛 Remove '__disable_customize__' parameters while queuing legacy emails Active . I will check it out soon once client work is less busy.

I did another try after converting all my custom modules to use Level 3 Native implementation of Drupal Symfony Mailer.
Everything works fine when I don't use Symfony Mailer Queue.

When I use it, it doesn't ☹️
The log of the cron job (Queue: Sends emails and retries failures) says:

Warning: Undefined array key 0 in Drupal\amap\Plugin\EmailBuilder\EmailBuilder->createParams() (line 37 of /var/www/html/web/modules/custom/amap/src/Plugin/EmailBuilder/EmailBuilder.php) @backtrace_string.
Warning: Undefined array key 1 in Drupal\amap\Plugin\EmailBuilder\EmailBuilder->createParams() (line 45 of /var/www/html/web/modules/custom/amap/src/Plugin/EmailBuilder/EmailBuilder.php) @backtrace_string.
TypeError: Drupal\symfony_mailer\MailerHelper::parseAddress(): Argument #1 ($encoded) must be of type string, null given, called in /var/www/html/web/modules/custom/amap/src/Plugin/EmailBuilder/EmailBuilder.php on line 72 in Drupal\symfony_mailer\MailerHelper->parseAddress() (line 101 of /var/www/html/web/modules/contrib/symfony_mailer/src/MailerHelper.php).

The good thing is that the correct EmailBuilder is called.
The problem is that the parameters of the email aren't sent from the queue.
Sorry, I couldn't debug further...