The IUPOP3 server is a VMS implementation of the Post Office Protocol
Version 3, based on RFC 1081 and somewhat on Berkeley's popper.

There are many differences between IUPOP3 and the popper, partly due to
the differences between VMS and UNIX as operating systems.  Probably the
most notable difference is that IUPOP3 is a static multithreaded server.
The server always listens on the network port for new connections, and
can handle up to 31 simultaneous POP3 client connections.  In this way,
expensive VMS process creation overhead can be avoided.  Note that this
does not mean that IUPOP3 can serve only 31 clients; indeed, at IU
it serves dozens.  The limitation is 31 concurrent connections from
POP3 clients at any given moment in time.

IUPOP3 was developed and tested on VMS 5.3 and 5.4 systems, using the 
VMS callable mail (MAIL$) interface.  The current release is compatible 
with the following TCP/IP network implementations: Wollongong WIN/TCP 
for VMS, DEC's UCX, and TGV's Multinet.