SMTP +Concepts

This section section describes the Simple Mail Transfer Protocol (SMTP) +and its implementation on Symbian platform.

SMTP protocol

SMTP +is the most widely used protocol for sending emails. It is a text-based and push protocol +that cannot pull messages from a remote server on demand.

One +or more recipients of a message are specified in a message along with the +message text and other encoded objects. The message is then transferred to +a remote server using a procedure of queries and responses between the client +and server. Either an end-user's email client (Mail User Agent (MUA)) or a +relaying server's Mail Transport Agents (MTA) can act as an SMTP client. The +SMTP client initiates a TCP connection to SMTP server on port 25 (unless overridden +by configuration).

SMTP is more suitable for a machine permanently +connected to the Internet. To retrieve messages only on demand, which is the +most common requirement on a single-user computer, a mail client must use +POP3 or IMAP4. POP3 is mostly preferred when single-user computers connected +to the Internet only intermittently.

Symbian platform implementation of SMTP

SMTP is +a Mail Transfer Agent (MTA) used to transfer emails to a SMTP server which +then forwards them on to a specified destination.

Sending an email +generally consists of three phases: connect and authorise to server, send +emails and quit.

The SMTP component is responsible for sending email +messages composed and stored in the Message Server’s Message Store.

The SMTP client initiates +a connection with the SMTP server. Depending on the user’s email settings, +this may have been initiated over a secure SSL/TLS socket on port 465.
Depending on how the +user has configured the email settings and the capabilities of the SMTP server, +SMTP authentication may also be required.
Mail From and +recipient To are sent.
The message header fields +for the email message stored in the Message Store are then transmitted, each +one separated by a CRLF.
The last CRLF is sent +to indicate the end of the RFC 2822 header.
The message body and +encoded attachments are sent.

The encoding format (uuencode or MIME, +character sets) depends on the user’s email settings and locale.
Finally, the body is +terminated by a CRLF.

CRLF sequence, and the email is queued by the +SMTP server for sending.

SMTP example

For more details on implementing the +IMAP4 MTM functions, see example +code.