Of course 90% of our contact form spam stopped the moment we installed the following php function into our form to mail scripts. That at least will give you a good idea of the industries that do this sort of thing.
function checkspam($variable)
{
$poowords = array("casino",
"penis",
"gambling",
"poker",
"escort",
"pharmacy",
"generic drugs",
"cialis",
"viagra",
"bcc:",
"content-type:",
"to:",
"cc:"
);
$variable = strtolower($variable);
foreach ($poowords as $val1) {
if (strpos($variable, $val1) !== false) return "true";
}
}
By the way, the reason for testing for bcc:, content-type:, to: and cc: is to stop the annoying buggers who try to hijack php mailer forms by injecting header content into form fields. A good practise for anyone programming forms is to make sure you test for and filter content that shouldn’t be there.