I'm a developer who works on that (type of) system for a living.
If you receive an automated marketing email from a dealership, they are _legally obligated_ to provide an unsubscribe button. We use SendGrid for that purpose, but I'm not sure who other companies use.
If there isn't an unsubscribe button, or you continue receiving emails after unsubscribing, you should be able to file a complaint with the FTC.