This is vm-pop3d. It is a POP3 server that can run as a stand-alone
daemon or it can run via inetd.

It has support for using extra password files and mailbox spool
directories for each domain. (It provides support for virtual mail
accounts.)

It can be used without any superuser privileges (as long as the
mailboxes are writable by the user/group running vm-pop3d).