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richard
richard

Reputation: 12526

Why do "user-level" client mail applications typically only use SMTP for outgoing mail (and not receiving mail)?

From here.

While electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) or a proprietary system (such as Microsoft Exchange or Lotus Notes/Domino) to access their mail box accounts on a mail server.

Why is that?

Upvotes: 0

Views: 417

Answers (2)

geekosaur
geekosaur

Reputation: 61439

SMTP was designed for use with "always on" servers; most people do not leave their mail client running 24x7 and able to receive email at all times. More recently it became possible to use ETRN to instruct a remote server to try to deliver anything queued up for a client, but this is still not widely supported and in general doesn't work as well as client-side "pull" (or IMAP's fake-push IDLE extension).

There's an additional issue in that most servers send warnings to the original sender when delivery to the next server fails; this is a good thing for intra-server transfers but a bad thing for intermittently connected clients.

Upvotes: 2

Craig White
Craig White

Reputation: 14012

On that same page it talks about it.

SMTP vs mail retrieval

SMTP is a delivery protocol only. It cannot pull messages from a remote server on demand. Other protocols, such as the Post Office Protocol (POP) and the Internet Message Access Protocol (IMAP) are specifically designed for retrieving messages and managing mail boxes. However, SMTP has a feature to initiate mail queue processing on a remote server so that the requesting system may receive any messages destined for it (cf. Remote Message Queue Starting). POP and IMAP are preferred protocols when a user's personal computer is only intermittently powered up, or Internet connectivity is only transient and hosts cannot receive message during off-line periods.

Upvotes: 3

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