We had to consider this when writing the nms formmail program. Formmails are the most badly abused CGI programs on the web. Here are what we recommend (and, whenever possible, enforce) in formmail.

• Only ever allow the form to send email to a fixed set of email addresses. Most spammers will test formmail programs by trying to get an email sent to themselves. If you don't send email to addresses that are input through the form then the spammer won't get his test email back and will probably abandon your site.
• If you're sending to ad-hoc email addresses, then only send fixed text that is configured in the program and not taken from the form. If a spammer can't change what the email says then your system is of little use to him.
• If you can't do either of the above, then force people to register (and check their email address is valid) before using your system to send email. That will discourage spammers as (for obvious reasons) they don't like to leave a trail.

As in so many areas of life, it's just a case of making your system harder to abuse than your neighbours'. If your system gives the spammers any trouble at all then they'll soon move on to a less well-guarded server.

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