Simple Description of SMTP protocol

In terms of email, we already know the POP protocol and IMAP protocol. Today, we will introduce the SNTP protocol. This Protocol also plays an important role in email. SMTP is a reliable and effective Email transmission protocol. SMTP is a mail service modeled on the FTP file transfer service. It is mainly used to transmit mail information between systems and provide notifications related to letters.

SMTP is independent of a specific transmission subsystem and only supports reliable and ordered data stream channels. One of the important features of the SMTP protocol is its ability to transmit mail over the network, that is, SMTP mail relay. Generally, a network can be accessed by a TCP host on the public Internet and a host on a TCP/IP network separated by a firewall, and other hosts in the LAN/WAN are composed of Non-TCP transport layer protocol. By using SMTP, mail transmission between processors on the same network can be realized, or mail transmission between a processor and other networks can be realized through a repeater or a gateway.

In this way, the Mail may be sent through a large number of intermediate relay or gateway hosts from the sender to the acceptor path. Domain Name Service System (DNS), the mail exchange server can be used to identify the next hop IP address of the sent mail.

Protocol Structure

The SMTP protocol command is the ASCII information sent between SMTP hosts. The command may be as follows:

Command	Description
DATA	Start information writing
EXPN <string>	Returns the name in the specified email table.
HELO <domain>	Return email server identity
HELP <command>	Returns the information in the specified command.
Mail from	Initialize an email Session on the host
NOOP	No response except server response confirmation
QUIT	Terminate an email session
Rcpt to <user>	Specify who received the email
RSET	Reset email connection
Saml from	Send email to user terminal and email
Send from	Send email to user terminal
Soml from	Send an email to your terminal or email address
TURN	Exchange roles between the receiving end and the sending end
VRFY <user>	Verify user identity