Some sites will play party tricks, which can get *some users* email
addresses.   Possible tell-tale signs of this are inordinate delays
loading a page (fingering @REMOTE_HOST - doesn't often work but
probably can't be detected from the webpage), or a submit button that
appears to do nothing at all (a mailto: form - works well with some
browsers but trivially detectable).   As a "snoop" party trick that's
fine, but if you find someone abusing these facilities (eg they send
you junkmail), alert their service provider!