WinProxy and Internal Mail Server

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I need some advise about how to do this and what software to use. I am operating WinProxy on my NT network, using my administrator computer as the access point to my ISP. I have about 100 employees and would like to set up e-mail. I already have Microsoft Mail set up on the internal NT LAN. What do I need to do to set up Internet mail? I want my employees to have e-mail addresses like "employee@company.com" Will I need to get more mailboxes from my ISP or can I make my own mailserver with software? Can I use Microsoft Internet Mail since I already have MS Mail set up on the network? Hopefully you can understand what I want. Thanks in advance for any information or links.