ASP.NET server-side controls postback to their own page. This makes cases where you want to redirect a user to an external page, but need to post to that page for some reason (
I would do the form post in your code behind using HttpWebRequest class. Here is a good helper class to get your started:
http://geekswithblogs.net/rakker/archive/2006/04/21/76044.aspx
From there, you can just do a Response.Redirect, or perhaps you need to vary your action based on the outcome of the post (if there was an error, display it to the user or whatever). I think you already had the answer in your question to be honest - sounds like you think it is a post OR redirect when in reality you can do them both from your code behind.