It can be done. How difficult it is depends on whether you need to implement the mail server (SMTP/POP3/IMAP) as well, or whether you just want to create a web-based mail reader/writer which talks to an existing mail system.
On the assumption that you are doing the simpler case, here's a rough recipe:
Ingredients:
A web browser
A working network or internet connection
A working SMTP server (for sending mail)
A working POP3 or IMAP server (for receiving mail)
A web server
Some way of running Java
Servlets (either built in to the web server, or an extra program)
The Java Activation Framework (Glasgow) jar file
The Java Mail jar file
A JavaMail plugin for POP3 or IMAP (whichever you use)
Steps:
1. Install the jars in the classpath used by the servlet engine
2. Write a few tests to make sure that you understand JavaMail
3. Write one or more servlets to send/retrieve/store mail
4. Install the servlets and away you go.
Diagram:
But before you get too deep into this, make sure you check out all the java code repositories, as I'm sure many people have done this before.
[This message has been edited by Frank Carver (edited March 07, 2000).]