Are you using client validation or server validation?
With server validation, the content is checked when the page is posted back. Where your action is to a different page, there's no opportunity to execute the server side validation.
ASP.Net 3 onwards supports "Client-Side Validation". This loads validation scripts into the web page which are executed in the client without posting back. The following is the MSDN article on client validation.http://msdn.microsoft.com/en-us/library/yb52a4x0(v=vs.85).aspx
A work around would be to post back to your asp.net page so you can validate the form. Then render a redirect page such as this:
<form action="LegayBay.php" method="post">
<input type="hidden" name="field1" value="value1" />
<input type="hidden" name="field2" value="value2" />