What do you mean by SMTP?

The SMTP (Simple Mail Transfer Protocol) is simply a mail Transfer protocol, a set of rules for sending messages from the source address to the destination to control how the letters are transferred. The SMTP protocol belongs to the TCP/IP protocol family, which helps each computer find the next destination when sending or relaying letters. With the server specified by the SMTP protocol, we can send e-mail to the recipient's server and the whole process takes only a few minutes. The SMTP server is the outgoing mail server that follows the SMTP protocol and is used to send or relay e-mail messages you send.

The user who sends the e-mail must also set an SMTP server address for the mail client to specify which server you want to send the message to. Our common ISP usually sets a computer to be both an POP3 server and an SMTP server. Of course, if the other side does not qualify IP range, you can also choose another ISP's SMTP server, the price is often slow connection.
Sending mail can also be done interactively via Telnet Port 25th and the SMTP server, first by confirming the native address, sending a command to confirm the email address and alias, and then sending the message, such as:
Telnet public.east.cn.net 25
HELO ppp25.east.cn.net
VRFY abcd@public.east.cn.net
VRFY pccomuting
MAIL from:

RCPT
To:
DATA
Hello,this is a test mail.
.
QUIT