SBS2000, OWA, and Proxy problems

Ok, to start I want to say this is more of a operational question then one of security.

I am running an SBS2000 server, and several of my remote users use OWA for email. Everything works fine, until one of them comes into the office and wants to use it. Then when they try to log on, it will not allow them too, until I disable the proxy/ISA server settings in IE. Once disabled it works fine.
But of course, I want those setting enabled, so ISA can give me the reports on usage. I've check and double checked all the settings and searched TechNotes, and found nothing that points me to any kind of a solution.

Any insights would be helpful!

Thanks!
MrC
/EDIT: I forgot to mention, everything else under the proxy works just as it should, no problem getting to allowed sites, and proper blocking on disabled sites.

Do they have the Firewall Client installed?


MLF

Yes, actually they do. But I do not think that is the issue, because all sites work, and the local firewall rules allow traffic on the proper port. I am beginning to think it is more of a AD security issue then a OWA issue.

Thanks for the suggestion, I will double check my firewall settings.

Yes it may be a AD permission thing..such as a Remote\Mobile user veruses a Domain User...or something like that.

Also...is it user specific...or machine specific???

MLF