A lot of times you have to submit an email address to sign up on a forum or to download a file. Some reputable websites claim they never give out or sell your email, most lie. They have a clause where they state they can give it to their partners - and as soon as their partners have it the sky is the limit.
Do I have any proof of this, yes. I use an email service to create a unique email anywhere I have to submit my email and I don't end up with a lot of spam this way. I have seen email sent to addresses I gave out to companies who claim they don't sell their members emails, but when you cancel service with them they have no problem selling it.
This technique acts like an intermediary between you and the spammer (possible spammer) by keeping your email hidden from them http://sneakemail.com/ "Sneak Email" have very good explanations on their service, it's free and they do not sell or give out your real email address. One drawback with using this service is the extra step to create a new email address each time and the other is "a lot of the professional (oxymoron statement) spammers" reject sneakemail created emails. If that is they refuse my sneakemail address I assume my email is not safe with them and I'll go on my merry way.
If you need a one time email and they don't accept sneakemail addresses create a yahoo or a hotmail account and write down the onetime use for it. I create addresses like nospam_fake_email1@yahoo.com and then I wait for the reply to my inquiry and I forget I ever used that address.
Anytime you need to give up your email, signing up for a subscription or enter a site to read an article use at least a free email, never use your "ISP's" email because as soon as they get it you end up on a spam list and you can get a couple of hundred spam a day.
Never, ever reply to a spam. Never ever click on an opt-out link it like hitting the fire alarm telling the spammers "We got a live verified email from a dummy who took the time to read and reply"
99% of the time the reply to email are a fake email or someone the spammer is trying to take revenge on using his or her email address as the sender. Opt-out links are opt-in to spam links. Than Can-Spam act that are supposed to protect us from spam is useless because most spam is sent using zombie or hijacked / compromised computers.
Nils