Step-by-step: Creating a suppression list that syncs across all our sending services.

Hey everyone, I've been tasked with setting up a suppression list for our company. We use a few different services to send emails (like Postmark for transactional and Customer.io for campaigns). The problem is, if someone unsubscribes or hard bounces in one place, they aren't blocked in the others.

I need a simple, reliable way to keep a master list that syncs automatically between all our tools. Budget is a concern, so I'm hoping for a DIY approach first.

What's the best way to architect this? Do I need a central database and an API for each service? Or is there a simpler tool that does this already? Any step-by-step guidance or common pitfalls would be super helpful! 😅