exchange server & remote outlook client

I have 2 pcs running NT6a and Outlook 98 with corporate option.The pcs both connect to the network via dialin through shiva modems. Connects go fine. Internet access is fine. Email isn’t fine. It will send to an “internal” address (defined on the exchange server) but will not logon to the server to retrieve mail. The error message is that there is no such mailbox on the exchange server. It is there. I added it. Also, any mail from or to anywhere but our exchange server causes an undeliverable message. Any ideas on what is going on? Thanks!