Try Contact Form 7, I'm pretty sure they use wp_mail(); . As this is a production site, you could install the plugin, then set the form up on a private page so you can test it in peace without customers stumbling across it...
Alternately, I use a variation on the form in this tutorial for my testing:
http://wp.tutsplus.com/tutorials/creative-coding/creating-a-simple-contact-form-for-simple-needs/
Just having a very basic form on hand to test things can be very helpful sometimes.