Spam check? - verify email address before email gets delivered.

Does anyone know where I could get one of those spam checkers which make people verify their email address before you receive their email?

I think you're thinking of BoxTrapper. It's pre-installed on most servers.

Nah just tried that and it's not quite what I'm looking for. It needs to be done via a webpage ie it sends an automatic reply to the sender saying "please click here to confirm your email address" and then sends them to a webpage where they have to click "Confirm".

I use ccNewsletter for Joomla, it features a double opt-in which is what I think you are looking for. It can be found here:
When someone submits their email, they are added to the list as "unpublished" until they click the activation link in the email they get when they sign up.

They don't actually have to click anything once they arrive back at the Website, but the mere fact that they bothered to click the activation link in their email is a pretty good guarantee that they actually want in to the list.

Wohoo! Found what I was looking for - Spam Arrest.