Trivial uses of Telnet - SMTP

SMTP - outgoing e-mail

SMTP stands for Simple Mail Transfer Protocol; this is the majority of the internet sends its messages and from time to time it is a very useful thing to know how this works. Standards suggest that the SMTP service should listen on port 25, although again it is trivial to reconfigure the service to listen on another port.

Commands required to use SMTP proficiently are very few, but to start using it "creatively" you need to understand what it's doing. Similar to POP3 the service uses text comments along side a status marker, however under SMTP these are more detailed status codes rather than a general state so all the computer needs to worry about is the codes - the text is simply there to make the protocol accessible to end-users accessing the service via direct connection. These messages take the form {code} {text} as you will see in the examples below.

• HELO - a way to set an identity to be used for sending this mail, mostly this is just for show as the service will automatically detect the correct name for yourself, since if this worked as expected mail would be so easy to fake. For the sake of complying to the specification it does need some text after it so that the command gets accepted.
• EHLO - another way to set an identity to be used for sending this mail, mostly this is just for show as the service will automatically detect the correct name for yourself, since if this worked as expected mail would be so easy to fake. For the sake of complying to the specification it does need some text after it so that the command gets accepted.
• MAIL FROM: - defines the e-mail address of the sender of the message.
• RCPT TO: - defines the e-mail address of a recipient of the message. Repeating this command once for each recipient means you can send one piece of mail to many users without having to repeat the entire process over and over again.
• DATA - marks the start of the data portion of the message, essentially everything that you would consider "content", this includes the "To:", "From:", "CC:" etc. as these are all simple components which your e-mail client picks out of the content and displays in a far nicer format. Just as a reminder - anything which is in the content can be faked as it is content and so consequently cannot be validated.
• .{ret}{ret} - this sequence of a period and two carriage returns marks the end of the data portion of the message, if you wish to use a period in a message normally you would use two periods.
• QUIT - closes your connection to the service.

Below is an example of how to send a very simple message via SMTP;

220 smtp.example.com ESMTP Sendmail 8.9.2/8.9.2/Debian/GNU; Sat, 9 Jun 2001 12:27:28 +0100 (BST)
HELO user123.example.com
250 smtp.example.com Hello user123.example.com [10.0.0.100], pleased to meet you
MAIL FROM: user123@example.com
250 user123@example.com Sender ok
RCPT TO: test@example.com
250 test@example.com Recipient ok
DATA
354 Enter mail, end with "." on a line by itself
Hello

.

250 MAA31024 Message accepted for delivery
QUIT
221 smtp.example.com closing connection


The next step along would be a basic e-mail which conforms much more to what a e-mail client expects an e-mail to look like. This example creates a plain text e-mail and sets MIME types, includes a neat version of the to: and from: data, uses a subject line and applies a content type before setting the header. From this example onwards I am not going to bother including server responses since they are all identical and just serve to confuse the information further.

HELO user123.example.com
MAIL FROM: user123@example.com
RCPT TO: test@example.com
DATA
MIME-Version: 1.0
From: Me <user123@example.com>
To: You <test@example.com>
Subject: Simple standardised email
Content-type: text/plain; charset=US-ASCII

Hello. Content goes here.

.

QUIT


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