Here's a super (I mean super) quick mvp of something similar to this idea that I just whipped up: https://isitgettingthrough.herokuapp.com/ . You can send/forward an email to three of the listed email providers with a special code in your email/newsletter and the website will check the inbox of each of those emails and let you know if the email came through. If it goes into spam or any other folders it won't show up on the page.

The issue however is that during some email outages only certain user(s) are affected, so this app cannot work 100% of the time if its account is not affected. If there's some more interest I can add some more email providers.