I've used a process for a while now, where I give a unique email address to every company with which I do business. (Email providers like Gmail make this relatively easy.) Normally, it just helps me sort my incoming messages. With any modern email program, it's fairly straight-forward to automatically filter messages that match certain criteria. You could keep order confirmations in one folder, file emails from clients separately, etc. (I won't bore you with the details.)
A few months ago, I voluntarily filled-out a form at the Tilted Kilt in Chicago to "help our waitress" enter a raffle or something. On the form, I provided an email address that was only given to that establishment.
Over the course of the next couple months, I received promotional materials from them. No surprise. They sent me a birthday coupon; they promoted their upcoming calendar; I completely expected this and didn't mind.
But just today, I received an unsolicited email at that same address from International Art Jewelers -- that just happens to be in the same building as Tilted Kilt! Suspicious?
I called the jewelry store, mentioned the email I received, and asked where they got my address. His immediate response was: "Have you ever visited Tilted Kilt?"
They sold me out! Despite a privacy policy promising otherwise, they did just that. They're not the only seemingly reputable company I've uncovered with this process. I'm looking in your direction: Audi, Focus Camera, Snap.com, ZoomInfo.com, and All of MP3.
My point wasn't to simply bash TK. Don't get me wrong – I love the place! My point was to warn you about the information you provide so willingly. Even though you might trust the company today, will you always trust them? (Computers don't "forget" information, simply because it's old. Are you proud of everything you did in high school or college? What would your current employer/client think?) Do you trust their friends? (If that company partners or is acquired by another company in the future, they will surely share information.) How do you know they won't violate your trust? In the case of email, maybe the harm isn't that great: We get a little more spam. What if it was a phone company that shared my calling logs? What if it were a search engine that shared my history? What if it were a bank that shared my buying habits?
