This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
That's a big undertaking. An excellent open source mail server written in Java already exists: Apache James. Check it out to get some ideas on what's involved. Its UI is based on Telnet, though, so no web technologies are involved.