Truth. All ephemeral/temporary/one-time mailboxes suffer from the same issue - once enough people start using it, the website owners take notice and it's blacklisted.

It'd be nice if you could create temporary <insert reputable domain here> accounts on the fly. User provides a captcha solve, your service uses this to create a random account & log in, user can view inbox or click 'open all links'. This wouldn't work with gmail because of SMS verification but would probably work on other domains and circumvents the above problem.

And not all sites honor the "+". Too easy to filter that out.