Apparently, The site is hosted through dropbox
Not exactly true. I copied the HTML from Gmail and pasted it into an HTML document, then uploaded it to Dropbox.
The site itself isn't hosted via Dropbox, but the email I got, I rehosted.
You probably signed up for some other site that gave out your information to third parties
That could be true, but I don't know what site it is.
Someone probably entered your email instead of their own
That might also be true as well, I think it's probably a friend who is trying to spam my email.