The Matt's Script Archive is very old, and it is well known that it is insecure. I'm surprised that you didn't read the warnings, as even Matt, the creator of Matt's Script Archive, now recommends that people not use that insecure script.
This script at nms sourceforge is secure: http://www.scriptarchive.com/nms.html. You may, of course, also use what Arkadij suggested. Here are some other tips to help keep form-to-mail actions secure:
You should not enable recipient addresses to be specified from hidden fields within the html form. Specify the recipient e-mail addresses only within the script itself. Putting them in the form is not secure.
The script should include a specification for valid referrer addresses or domains. Referrers from anywhere else should be rejected.
Do not name the script anything obvious: don't use words such as "formmail", "form" or "mail". If it has already been named something obvious, rename it immediately. Name it "macktruck" or "halibut" or something else each client coins for himself.
I am not WHB staff and I am not paid.
I provide help in these forums on my own time.