Just a note as to unexpected e-mails. Like junk mail of the paper variety, lists are sold/traded between companies and information brokers. Usually, this is on the terms of use in your registration for the original site you registered to - often phrased as "business partners".
Contests, websites, "survey sites" are all just big data collection devices. I'm sure I am not the only one who came across the "free quilt patterns" that were 45 minutes of data entry, leading, not to free patterns, but to spam.