SMTP命令

简单邮件传输协议(SMTP)是ASCII协议。它基于客户端-服务器模型。它为此服务使用TCP端口号25。因此发送电子邮件；通过对建立TCP的源计算机进行哈希处理来将源IP地址从源传递到目标计算机。若要发送邮件，系统必须包含客户端MTA，并且要接收邮件，系统必须具有服务器MTA。 SMTP将该消息从客户端MTA传输到服务器MTA。要发送邮件，SMTP使用两次时间：一个在发件人和发件人的邮件服务器之间，另一个在两个邮件服务器之间。

SMTP命令：
这些命令从客户端发送到服务器。每个命令都包含一个关键字，后跟零个或多个参数。这意味着也有一些不包含任何参数的关键字。命令格式如下：

Keywords : argument(s) 

下表提供了14种不同的SMTP命令：

S.No.	Keyword	Command form	Description	Usage
1.	HELO	HELO	It provides the identification of the sender i.e. the host name.	Mandatory
2.	MAIL	MAILFROM :	It specifies the originator of the mail.	Mandatory
3.	RCPT	RCPTTO :	It specifies the recipient of mail.	Mandatory
4.	DATA	DATA	It specifies the beginning of the mail.	Mandatory
5.	QUIT	QUIT	It closes the TCP connection.	Mandatory
6.	RSET	RSET	It aborts the current mail transaction but the TCP connection remains open.	Highly recommended
7.	VRFY	VRFY	It is use to confirm or verify the user name.	Highly recommended
8.	NOOP	NOOP	No operation	Highly recommended
9.	TURN	TURN	It reverses the role of sender and receiver.	Seldom used
10.	EXPN	EXPN	It specifies the mailing lit to be expanded.	Seldom used
11.	HELP	HELP	It send some specific documentation to the system.	Seldom used
12.	SEND	SENDFROM :	It send mail to the terminal.	Seldom used
13.	SOML	SOMLFROM :	It send mail to the terminal if possible; otherwise to mailbox.	Seldom used
14.	SAML	SAMLFROM :	It send mail to the terminal and mailbox.	Seldom used