This article is from the LAN Mail Protocols FAQ, by John Wobus email@example.com with numerous contributions by others.
These protocols were designed and implemented mostly by
Internet-connected universities with some participation by other
Internet-connected research institutions. They were certainly devised
to handle the type of electronic mail that universities must do. A
typical site has probably 10 to 10,000 desktop computers and has an
Internet connection and also runs Unix, giving them the Unix sysadmin
expertise that makes running a Unix-based server attractive. Most of
the servers listed here run under Unix though some run under other
large systems and as time goes on, we are seeing more servers that run
on PCs and Macintoshes.
A more recent use of these protocols has been by Internet Service
Providers and their customers. Internet Service Providers require a
way to offer e-mail services to however many customers they provide,
to customers who are connected to the network only part of the time.
Like a campus application, they may have 10 or 10,000 customers to
serve. These protocols offer a distinct advantage over SMTP for such
purposes and form an attractive complementary e-mail service for WWW