Spam mail and Address Spoofing

Is is possible for anyone to sniff your e-mail address from a secure web site, say during a registration process on that site, and then spam you with a spooffed address making it look like it came from the site you were visiting? Or is it more probable that the site you enterned your e-mail address on has sold your address? Any insight into the minds of these Spamming #%$@’s would be greatly appreciated.