Formmail scripts are subject to hijacking if they accept input from an HTML form and that input includes the email address of the person(s) to which it sends email. So, if in your HTML form, you specify the "To:" for the email, it is subject to hijacking.
You can overcome this by editing the formmail script to specify a list of acceptable email addresses. Then, have the script check the "To:" against this list. If there are any outgoing emails where the email address is not on this list, have the script die.