[squid-users] authenticating OL and OE users

I want to configure Squid so that if a user uses OL or OE to connect to a mail server outside our network, that the authentication screen pops up. But if they connect to the local mail server, no authentication is required.

Currently, if users access the Internet, or their external mail accounts through a browser, the authentication screen comes up as it should.

I added the following lines to my squid.conf, but so far it still isn't requiring authentication from the mail clients:

acl local_mail dst 192.168.0.238
acl mail_client browser Outlook
acl GetPass proxy_auth REQUIRED
http_access deny !GetPass
http_access allow !local_mail

If the !local_mail comes after the !GetPass, shouldn't Squid require authentication?

Thanks.

Brian