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◅ POP3/SMTP - Whats the difference?

POP3/SMTP - Whats the difference? ▻

POP3/SMTP - Whats the difference?

In article <>, Dr Malcolm Norris
<> wrote:
>If the heat has now died down a bit, would anyone care to summarise in
>simple and unemotional terms what the differences are?

Other than that both are used to shift STD 11 (RFC 822) mail messages
around, they really are chalk and cheese.  It is facile to claim that
one is better than the other.

SMTP (STD 10, RFC 821; see also STD 3, RFC 1123, and RFCs 1651-1653) is
an MTA-MTA protocol, in other words it is used for moving mail between
two Mail Transport Agents.  (An MTA is part of the email "Post Office";
MTAs move mail messages around without looking inside them.  Examples of
MTA implementations are sendmail, PP, MMDF, PMDF, smail.)

An SMTP transaction is initiated by the MTA which holds the mail---this
is a problem if you want to use it over an intermittent connection.
Clearly any ISP using SMTP over intermittent connections will have
solved this problem.  (Here at PIPEX, we detect when an ISDN / PSTN
dialup customer connects by various methods---some automatic, some
requiring action from the customer's end---and then "kick" our MTA to
deliver their mail.)

SMTP deals with domains; a single SMTP transaction will normally
transfer all mail for a given domain, with the sending MTA paying no
attention to what's on the left of the `@' sign.  SMTP transactions are
not authenticated.

POP (RFC 1725) is a split-UA protocol, in other words it is used for
moving mail between two parts of a mail User Agent.  (A UA is the user's
interface for reading and sending mail.  Examples of UA implementations
are elm, pine, Eudora, MH, Mail-it.)  The "split" is between the front
end, which typically resides on the user's desktop computer, and the
POP server, which is normally coresident with an MTA, perhaps on a Unix

A POP transaction is initiated by the user's front end, and so POP can
be used "out of the box" over intermittent connections.

POP deals with maildrops, which normally correspond to a complete
Internet mail address (e.g. <>); a single POP
transaction will normally transfer some or all mail for a given
maildrop.  POP transactions must be authenticated with a maildrop name
and password.  Of course, an individual can have many POP maildrops.

POP can only be used to retrieve mail messages which have been
"delivered" from MTA-land to a UA (remember that the POP server is part
of the UA).  It offers no way to "submit" a message from a UA to an MTA.
POP-based UAs typically use SMTP for message submission.

Just to confuse things further, there is also the Internet Mail Access
Protocol, IMAP (RFC 1730).  This plays a similar role to POP, but offers
richer functionality.

Tim Goodwin        | "If you're used to paint-by-number, a blank canvas
Public IP Exchange | can be unsettling at first." -- Larry Wall

Original headers:

From: (Tim Goodwin)
Subject: Re: POP3/SMTP - Whats the difference?
Date: 19 Jun 1995 13:32:23 +0100
Organization: PIPEX, 216 Science Park, Cambridge, England
Message-ID: <3s3qon$>
References: <> <> <3rpscn$> <>

△ △

◅ POP3/SMTP - Whats the difference?

POP3/SMTP - Whats the difference? ▻