Lot's of mail-scripts suffer from lack of anti-spam support.

FormHandler has several techniques to prevent spammers from using your form
and scripts to sent spam.

The first thing you need to do is only allow your own hostname or
ip-address to use the formHandler script.

In the future you can use the getkey.php script in your form to send MD5
checksum keys to the formHandler script to prevent hostname or IP spoofing.
This code is in development right now.

It will not totally prevent spamming, but people have to download and
install the script to see how it is working.