vm-pop3d is a POP3 server. It is free to download and use. It supports
alternative password files and mail spool directories; it can be used for
setting up virtual email accounts -- mailboxes without real Unix owners for
each. This will allow you to have multiple email accounts with the same name
on one system. 

WWW: http://www.reedmedia.net/software/virtualmail-pop3d/