That’s a problem as described above. What needs to happen, I think, is to add a webhook for mail jet so that mail jet can notify discourse when this happens. (there is a howto for mailgun and others).
Though having mall jet remove their unsubscribe links is good, it still doesn’t cover the case when an address stops working for some other reason.
I recently added a PR for postmark: Postmark bounce support? - #6 by pfaffman.
I’d imagine (but it’s not my decision) that a similar PR for ElasticEmail would also be welcome.
Their webhooks are documented here: https://help.elasticemail.com/en/articles/2376855-how-to-manage-http-web-notifications-webhooks