outlook web acces deriere un proxy

Bonjour,
J'ai un Proxy(smoothwall) pour fournir mon Internet au bureau.
j'aimerai rendre disponible OWA depuis Internet.
j'ai fait un port fowarding. sa marcher pour les site web du serveur sauf
http://monproxy/exchange
il me demande mon authentification. et après il me donne un page


The page cannot be displayed
The page you are looking for is currently unavailable. The Web site
might be experiencing technical difficulties, or you may need to adjust your
browser settings.

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Please try the following:

a.. Click the Refresh button, or try again later.

b.. If you typed the page address in the Address bar, make sure that
it is spelled correctly.

c.. To check your connection settings, click the Tools menu, and
then click Internet Options. On the Connections tab, click Settings. The
settings should match those provided by your local area network (LAN)
administrator or Internet service provider (ISP).
d.. If your Network Administrator has enabled it, Microsoft Windows
can examine your network and automatically discover network connection
settings.
If you would like Windows to try and discover them,
click Detect Network Settings
e.. Some sites require 128-bit connection security. Click the Help
menu and then click About Internet Explorer to determine what strength
security you have installed.
f.. If you are trying to reach a secure site, make sure your
Security settings can support it. Click the Tools menu, and then click
Internet Options. On the Advanced tab, scroll to the Security section and
check settings for SSL 2.0, SSL 3.0, TLS 1.0, PCT 1.0.
g.. Click the Back button to try another link.



Cannot find server or DNS Error
Internet Explorer

merci de votre aide future.
Sébas