It depends on the look and the feel of the Website, and the product itself. It also depends on where you put the box. Many times you have to literally put it right in front of their faces, and get them to sign up. It can range from 5%, on up to 25%. Who knows. But mostly it is under 10%. A lot of people just aren't into that much reading on the Net. We are a nation of skim readers. If you have a squeeze page where one has to sign up just to get into the site, you will get one hell of a lot of fake addresses. I do that. Example: . I'll type gibberish in. ~ And then you have the ones who stop reading after the second or third letter. But go for it. Test it to see for yourself. Post your results.
One way to get around that problem is to have the link to the website that they are trying to get into contained in the 1st email you send out, and not have the access page on the redirect after signup (and make sure this is clear on your signup page). This should make it clear to anyone signing up that they MUST give a real email address to get to the information they want!
That's good advice, InformationEmpire. But if one has to do that, just to get readers, I don't know, man. I know we are talking newsletters here, but I don't know how many are out there who has the same thinking as mine...but if I have to give my e-mail, JUST to get into a site, I'd pass. I don't know...I guess it's different strokes for different folks.