How does one conditionally forward POST values to another page?

Specifically, let's say I have a page with a form on it where a user clicks Submit.  This page checks the input fields when the user presses that button.  If the fields contain mistakes, the appropriate fields' labels are turned red.  That much is easy.  Let's say if it's right, I want to POST that form to an external page written in another language.  I cannot use any .NET conventions for transferring data, so it must be POST values.  The postback attribute would seem to be for this, but I need the forwarding to be conditional.  Eg:  They're only posted to the external page IF my button handler determines that the values are good.


Thanks! 
...