Is it possible to replace the standard emails with webhooks, so i can use my own email service?

If you are talking about system emails under auth settings.

I know about this, but i want to use my own service on my server utilizing Postmark templates. But i want Supabase to send my server a webhook request whenever a user signs up etc

You can certainly have a trigger function on insert to auth.users and call a service with the http extension or function hooks