Outlook Anywhere Password Prompts - Only on certain external networks

Question

I am running a standalone Exchange 2007 SP3 server on Windows Server 2008.  I have published Outlook Anywhere via ISA 2006.  Outlook Anywhere is configured for Basic Authentication.  All clients are using Windows 7 with Outlook 2007 with latest
service pack.  This is not a new configuration, I have been running this for quite some time.
I have a strange issue going on.  The issue is that Outlook Anywhere users receive a prompt for their username and password ONLY when they are connected to certain external networks. Users began reporting this several months back.  It happens on
all mobile users that I have tested with. 
Basically, what happens is a user takes their domain joined laptop out to another work site.  This site is not connected to our network.  The IT department of the site connects my users laptop to there network.  User then starts Outlook, Outlook
connects via HTTPS as it should, down in the bottom of Outlook it shows that it is connected to Exchange.  However, within usually about 30 seconds the password prompt comes up.  User puts password in and clicks remember and OK, but
the password box comes right back up.  Sometimes it will except the password and run for a little while but then prompt again.
If I check Outlook Connection Status it displays that connections are established via HTTPS as they should be.  Latency isn't too high averaging about 150 - 300ms.
If the user clicks Cancel instead of entering their password, Outlook will continue to run and it can send and receive email. Connection status still will show connected. However, if the user opens the address book and tries to access one of our
Address lists other than the GAL, then Outlook displays a message stating the user doesn't have permission.  If the user clicks need password at the bottom and then enters their password at the prompt the address book will work. 
This happens at several work sites, each different networks.  My first thought is some sort of firewall issue at the sites but the IT at the sites say there should be no firewall blocking going on.
I have tested probably 5-6 other wifi networks, both public and private and Outlook Anywhere works perfectly on everything but these few work sites.
I have used testexchangeconnectivity.com at the sites at it tests fine.  I have cleared the cached credentials from "Manage Windows Credentials". 
Any assistance is appreciated.

Answer 1

I am running a standalone Exchange 2007 SP3 server on Windows Server 2008.  I have published Outlook Anywhere via ISA 2006.  Outlook Anywhere is configured for Basic Authentication.  All clients are using Windows 7 with Outlook 2007 with latest
service pack.  This is not a new configuration, I have been running this for quite some time.
I have a strange issue going on. The issue is that Outlook Anywhere users receive a prompt for their username and password ONLY when they are connected to certain external networks. Users began reporting this several months back.  It happens on all
mobile users that I have tested with. 
Basically, what happens is a user takes their domain joined laptop out to another work site.  This site is not connected to our network.  The IT department of the site connects my users laptop to there network.  User then starts Outlook, Outlook
connects via HTTPS as it should, down in the bottom of Outlook it shows that it is connected to Exchange.  However, within usually about 30 seconds the password prompt comes up.  User puts password in and clicks remember and OK, but
the password box comes right back up.  Sometimes it will except the password and run for a little while but then prompt again.
If I check Outlook Connection Status it displays that connections are established via HTTPS as they should be.  Latency isn't too high averaging about 150 - 300ms.
If the user clicks Cancel instead of entering their password, Outlook will continue to run and it can send and receive email. Connection status still will show connected. However, if the user opens the address book and tries to access one of our
Address lists other than the GAL, then Outlook displays a message stating the user doesn't have permission.  If the user clicks need password at the bottom and then enters their password at the prompt the address book will work. 
This happens at several work sites, each different networks.  My first thought is some sort of firewall issue at the sites but the IT at the sites say there should be no firewall blocking going on.
I have tested probably 5-6 other wifi networks, both public and private and Outlook Anywhere works perfectly on everything but these few work sites.
I have used testexchangeconnectivity.com at the sites at it tests fine.  I have cleared the cached credentials from "Manage Windows Credentials". 
Any assistance is appreciated.