[tialaramex]

The practical entropy of email addresses is indeed pretty small, lots of them are going to be [e-mail] and a bunch more end in gmail.com or another popular provider.

If you can accept some level of false positives you could make the hash too narrow to be able to usefully reverse it. For example if only sixty people will ever subscribe or refuse to subscribe,a 24-bit hash is plenty to reject mistaken attempts to subscriber somebody who doesn't want in, but good luck guessing which GMail user is "2ca24b".

Another problem is, what if the email address changes hands - maybe even the whole email domain changed ownership. You probably need a way for people to change their minds, as that then also covers the case where the person behind the address changed.