http://www.independent.co.uk/life-style/gadgets-and-tech/cri...
>Some light digging unearthed the fact that the company responsible for this particular dark art is called Criteo, and the MD, John Buss, was happy to talk me through this relatively new technique. It's all legal, all cookie-based, but crucially its database – which learns about me and how I click around websites – anonymously links up with databases of email addresses it has licensed from other companies operating online. A match between the two, along with an assessment that I might be likely to splash some cash, triggers an email.
>How did my email address end up in the database in the first place? Well, at some point – and goodness knows how, because I do my utmost not to – I opted in to receive some communications "from selected third parties".