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SMTP

The┬áSimple Mail Transfer Protocol is used for sending email messages–desktop clients only use SMTP for sending messages–mail servers, on the other hand, use SMTP for both sending and receiving email from one server to another. Traditionally, SMTP operates over TCP port 25, though many ISP providers block port 25 to prevent spammers from sending messages, so other ports are sometimes used, especially when SMTP is sent over a secure SSL channel.

You can use a command-line telnet client to send an email if your mail server doesn’t require special authentication or encryption. For example, here’s the commands you would type when connecting… which are the same commands your desktop email client would send to the mail server behind the scenes.

HELO
MAIL FROM:<me@example.com>
RCTP TO:<test@example.com>
DATA
Hello there! This is a test message!
.
QUIT

As you can see, you can type in anything you want, which means in the old days, you could forge emails from anybody, which led to a lot of fake emails from Santa Claus or other fictional characters… and then a lot of spam. Most servers require authentication these days to prevent that problem.

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