[kretor]

There's a third possibility: The advertiser guessed the email address.

[bgeeek]

Create unique email addresses with a password generator and don't distribute them. I don't buy into they just guessed them, tbh.

[reaperducer]

There's a third possibility: The advertiser guessed the email address.

Well, in this case probably not. Though I wrote "[e-mail]" above, it's really more like "[e-mail]." So the chances of it being guessed are infinitely small.