SMTP is really only useful to deliver mail to multiuser hosts which
are permanently available and connected to the network. It is not
normally used to deliver mail directly to, for example, a user's PC
or Mac desktop system.
The Post Office Protocol (currently POP3) is
designed to allow mail to be delivered to a mailbox on, eg, a Unix
host using SMTP, but to later (at the recipient's convenience)
download the contents of the mailbox to their desktop system.
A POP client (eg Eudora, Netscape Mail, MS Outlook) establishes a
TCP connection (on port 110) to a server process on the (eg) Unix
system where the mailbox resides. The user is
authenticated (username/password), and the
contents of her mailbox is downloaded for processing on her PC or
Mac.
POP is almost universally used where a user has "dial up" Internet
access from a commercial Internet Service Provider - the user's
mailbox is maintained by the ISP. The IMAP
protocol has superior functionality to POP, but is not (yet) in
wide use.