accessing web site with proxy server

[Mark]

I have an asp.net application that when accessed by users not using proxy
servers there are no problems.
When a user using a proxy tries to access the site they access the site ok
but when they try to do anything on the site that causes a POST they error
out. I can't describe the error because there are many components to our
site including reverse proxies, netegrity siteminder authentication, other
stuff.. So, the error is just a custom error page that doesn't display
anything.
I did some basic tesitng with the user. the first thing i did was put a
blank asp.net page with a basic button that posts to itself. the user could
access this fine and hit the button no problem. The second test i did, was
added some fields and graphics to the pages and made the page query a sql
server DB and this also was fine. The third test was having our full blown
page that posts around 400 K of data when a button is clicked. when the
user clicks this button, it generates an error. When the user doesn't use
the proxy server for internet access they have NO problems with our site.
Any ideas as to what could be going on? This is happening with several
clients, all of whom have different proxies. Are there limits that proxies
can request? I'm out of ideas.

thanks for any suggestions.