Linux based E-Mail w/ Windows Clients

An Anonymous Coward wrote in with this question: "I have a Linux server with 15 Windows clients. It's currently connected to the net with IP-Masqurading. Right now we have the Windows machines directly connect to the ISP to get their mail. I want to have them connect to the Linux server and then use fetchmail on the server. Each computer has it's own email account, someguy@somedomain.com I'd like to be able to have fetchmail grab all of the mail for all of the accounts and then use something like qmail to allow the Windows machines to get their mail. Whats a good way to do this? Note that I *must* be able to have email accounts with long names, about 15 characters. And I'd like it to be able to automatically add any new accounts. "