Accessing Exchange from another domain (network)

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Hi. We have a couple of traveling users, that have laptops. They connect from the field through OWA with no problem. One of them recently was assigned temp. to a client and was given from them a desktop to use. He says the company uses Exchange and Groupwise for their email. I walked him through the settings by making a new profile, that points to our server, he added the server name and IP to the HOSTS file, and he can ping the server and access the server with IE. The problem is when he puts in his name in the Mailbox: field, and clicks on "Check Name" the computer sort of freezes, and eventually he gets a box that reads: "The name could not be resolved. Network problems are preventing connection to the Exchange Server computer..." This also happens if he puts in the IP address, instead of the Server name. What is happening? The only thing I can think of, is that Exchange uses a port that is not open in the network he is at. What ports should I tell the Administrator at the other end to open so that this guy can access his memos and stuff? Thanks for your help. Click.