Multiplexing of incoming mail has is covered by multiplexing SMTP, as this is the only protocol commonly used to distribute email on the internet. If other protocols were to be used for mail delivery, then these could be passed through an SMTP gateway in any case.
Multiplexing of mail retrieval is more complex. In my experience, POP3 is overwhelmingly the most popular method of mail retrieval. Other methods that are commonly used are IMAP and direct access to a mailbox using shell access. It is of note that perdition can easily be extended to work with TCP/IP connection oriented protocols.
Users who wish to access their mail using shell access can be
assigned to a single server and given shell access to the spool
directory, possibly via a network file system accordingly.
Another approach to offering shell access is to develop a method
of transparently transferring mail mail for shell users into a
local mailbox.