Exchange 2000 OWA Problem

I have a client that would like to use Outlook Web Access for remote mail. That part of OWA works great. They enter the URL to get to the login for Exchange 2000 then their mailbox, calendars etc. are dispalyed. The problem is that they use several dozen public folders. Which aren’t currently showing up. When the user logs on to OWA in the building they get the public forlders displayed. When they go to another site (their home for example) the following error pops up while trying to access the folders: The server name or address could not be resolved 12007. OWA continues to work except they can’t access the public folders.

Is this Firewall related? The only ports opened are 80 and 25.