Searching for comment email address with a custom form using POST action

I have created a page with a custom input form that uses itself as the form action that shows a table of results based on posts with a specific email address attached to it. Users should be able to search any email address and find such posts. However, I am unable to manipulate the input string when it properly includes an @ or at symbol.

Using Firebug, I can see that the browser detects the email address in a POST variable, yet WordPress does not recognize an email address in the POST variables.

My form only has one input field for the email address but isset($_POST['textinput']) always returns false!