I suspect Poloniex, but can't be sure. I received those to 2 different email addresses. I use one of them for multiple purposes (registrations on various website etc), the other one is more 'official' and I rarely used this one for any registrations (submitted it to maybe 3-4 established sites), I don't recall receiving any bitcoin related spam to this email before.
Both emails were used to register on Poloniex (I have 2 linked accounts), hence my suspicion.
@OP - are you registered on Polo with this email address?
Edit: I also found similar email in yet another email account of mine, so Polo may not be to blame after all.
Yes, I have registered on Poloniex using the same email address and thus probably your suspicion is true.
I have not maintained different mail id's for different purpose so probably the email address was leaked out of 100s of websites on which I have registered.