Could be that they have access to the Bitfinex user database 
That is the only exchange i am registered with the exact mail address they sent to.
I received that mail just a few hours after i registered at Bitfinex.
That is the only exchange i am registered with the exact mail address they sent to.
I received that mail just a few hours after i registered at Bitfinex.
coindidence, likely.
They have been spamming this nonsense for several days now, and to a lot of completely unrelated addresses that never were used on any exchange.
They are spammers, not hackers.