There are obvious things like, they signed expecting options / functionality, or the email service they use is binning any service / activation emails. What's

There are obvious things like, they signed expecting options / functionality, or the email service they use is binning any service / activation emails. What's less obvious is possibilities such as your sign up html / web code is more robust ... but your service is only compatible with the latest and greatest browsers ... cough Netscape 11 issues, which I have witnessed frequently having an older system. Amazingly I don't cry and look to see how much money is in my wallet to upgrade so I can be part of the jet set ... actually not amazing, I'm just lazy and comfortable.