Please show us the code on exactly how you are doing this. Again, I don't think there's any need to URL encode the data as it is getting posted and not place in the querystring. What does URL encoding the data have to do with two systems in two different places?
Another though I had last night is that within the recipient ASP page you can not reference the form field by it's ASP.NET server control ID. You have to use something in a format like:
Request.Form("ctl00$ContentPlaceHolder1$TextBox1")
Look at the HTML source of your ASP.NET form to find the exact name you should use for the textbox.