The Simple 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. Hello
there! This is a test message! 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 req