Wholly is right, it won't take long and you will get spammed so bad you will need spamdex. lol.
Seriously, you will get abused. You are better off creating a web form and allow it to check for abuse before the message gets sent to your email program. Besides a web form is really more convenient for your users as long as they are not too limited in what they need to tell you. A word of warning, allowing a user to change the subject line can be used against you.
I ended up not using headers at all and placed all the header info into the message and the form used default headers so I knew it came from my website.