I send out a newsletter to customers about once per quarter. All recipients have opted in by actively checking a checkbox. I'm pretty sure these mail services

I send out a newsletter to customers about once per quarter. All recipients have opted in by actively checking a checkbox. I'm pretty sure these mail services already monitor unsubscribe rates and spam flags. But there will always be false positives, i. e. legitimate recipients flagging a newsletter because they no longer want to receive it, or because they forgot they agreed. Spam recipients, on the other hand, may not report these messages often enough. Some just don't bother, others rely on their mail client or company's filters. That could make it difficult to find a reliable cut-off separating legitimate and spam mail. The only real solution to this would be for the mail services to handle opt-in procedures. That, however, would effectively lock you in to a single provider, because it'd be suicide to change providers and ask for confirmation from every recipient again.