I finished coding Webhook Postmark support.
Event webhooks (Bounce, Spam plain, SubscriptionChange) from Postmark to Mautic are supported with this change.
I'm using this code in production with Mautic 4.4.2 and it works like a charm.
I share here the portions of code to modify, I hope it will be useful to someone:
in file: /mautic/app/bundles/EmailBundle/Config/config.php