problem with form post mailto IE7

Project Description:
I have a small but vexing problem with a website. My form post routine works normally with some IE7 browsers but not others. In some cases when the user "submits" the form, instead of sending the form by "mailto" (action=mailto: . . . ) and onsubmit redirecting the user to the "thank you page", it opens an e-mail window. I can't figure out if this is caused by a bug in IE7, or a security setting, an html problem, and issue with redirecting, or what? Somebody help! I have uploaded two files: the page containing the form and the thank you page. I assume this is something brief and simple, low budget!