How many email subscriber lists do you have? How do you keep them in sync?

Hey everyone! Does anyone have some great tips regarding how to do proper email unsubscribe management? Been struggling with the following at the startup I work for:

• we have 4-5 lists of "subscribers" between multiple SaaS products that send emails somehow: Intercom, Sendgrid, Mailchimp...
• users unsubscribe from one system, but are still subscribed in others
• when users then receive yet another email through some other system, they're angry - and rightfully so

Any tips on how to keep these in sync? Have you encountered this problem before?