when we using external OWA 2010 to an Upload an attachment we have error message(OWA attachment upload slow/fail external )

"The
action you tried to perform couldn't be completed because there's a
configuration problem on the server. If the problem continues, contact your
helpdesk. "     What is the reason ? how i can fix it ?

Please Note :  when using the OWA 2010 sp1  internally its working fine only from outside we facing the issue.

Thanks for your help