If they do not want to receive it, then typing some spurious addy, like a@b.com, will satisfy most simplistic checks. I don't know which poor blighter has the email addy a@b.com, but they must recieve a sh^H^H lot of junk they never asked for.

A lot of people don't seem to know about m!^example\.(?:com|org|net)!. These domains are specifically meant for documentation and such, and therefore are perfect for fake email addresses (foobar@example.com for example). There are no MX records for these domains, nor is there anything listening on port 25. So as long as you're giving this value to a program that doesn't check for an MX record, these domains are perfect -- they are fake, and at the same time, you're not chancing giving someone's real email address and getting them spam'ed to death.