Amen. Without a robust delivery testing program, there's no way to know whether emails are actually landing in the inbox of actual users.

37signals has an easier time of it because the vast, vast majority of their emails are transactional, so users will pipe up and complain if they are not getting them. Marketing emails are a lot harder to troubleshoot.

And before the masses get up in arms about spam, when I say "marketing" I am talking about even the most compliant, user-friendly, double-opt-in email campaigns. Even if people happily sign up for your emails, that doesn't mean they will tell you if they stop receiving them.