Posted by Rasmus on 09/28/74 11:43

Yeah, referer alone is a bad idea as some browsers don't support this or
have it removed with some software. I suggest making a spam score system
that rates every e-mail sent from your form and rejects mails with a spam
score larger than X...

Some ideas to use for rating:
look for undesired words (or count them to calculate a score)
look for desired words to lower the spam score
undesired top level domains of the poster
missing referer
"?" in $_SERVER['REQUEST_URI']
$HTTP_SERVER_VARS['REQUEST_METHOD'] != "POST"
$mails_today > something
....

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