This is gonna be like a small info bulletin on how email is sent.
Copyright © 2000 by Philip S Tellis
When one sends mail using the SMTP protocol, one has only the following commands:
All other commands simply extend the usage of these. There are also commands that come under the ESMTP protocol (Extended) that I will not cover here.
Looking at the commands above, one notices a few things:
I'll cover point 5 in a different discussion, but for now, let's look at points 1 through 4.
HELO, MAIL FROM: and RCPT TO: tell the SMTP server all it needs to know to get the mail through, and bounce it in case of error. HELO isn't really required, but ensures that the client and server are speaking the same language. (Ref: EHLO)
DATA tells the SMTP server what the contents of the mail are. The contents of /var/spool/mail/$USER or equivalent will show what was typed as DATA, with maybe a few lines added by the SMTP server and mail client.
Escaping a . is the same as escaping a \ in C. Preceed it with another . In fact, the SMTP protocol states that any line that starts with a . should be preceeded by another . So if I wanted to say this:
. is a period
I would have to enter this:
.. is a period
The MUA is supposed to translate .. at the start of a line into .
Sending a mail directly using the SMTP protocol will produce a mail with only the From: and Date: header fields set. To enter other fields, they must be entered as the first part of the DATA, separated from the actual body by a completely blank line. Not even white space is allowed on this line. eg:
DATA
354 Enter Data end with .
From: philip@mymachine
To: linuxers@ilug-bom.org.in
Subject: SMTP - [informational]
This is gonna be like a small info bulletin on how...
...
.
250 Mail accepted for delivery
When the mail is received, the MUA separates the header from the rest of the body.
One might ask why the To: header field is not included. On the face of it, it seems logical that the To: header should be included along with the From and Date as default headers, but think closely about this. When sending a mail, recipients may be in the To: list, Cc: list and Bcc: list. The To and Cc list behave the same way, but the Bcc list is a list of all recipients whose addresses must not show up in the mail header. As already noted, all recipients are specified using the RCPT TO: command, and there is no way for the SMTP server to distinguish between different types of recipients - nor should it care. It is the responsibility of the origin MUA to properly format the mail header, including only those headers that should be included, and excluding those that shouldn't.
This should in theory always work, except that some old MTAs messed up, and included the To: header if none was specified. To work around it, many MUAs add a To: Undisclosed Recipients if no recipients are specified. This is done at the sender's end, not at the recipient's end.
Now that we've had a look at the SMTP commands, and what each does, let's look at return values. All SMTP responses are three digit numbers followed by a single white space and an optional text message.
Multiline responses are signalled with a hyphen as the fourth character instead of a space. Every line of a multiline response starts with the three digit code and a hyphen. The last line has a space as the fourth character. As a general rule, a space as the fourth character signifies end of response.
On connect, an SMTP server responds with a 220 if it is able to serve the request. Any other return code should be considered an error.
HELO, MAIL FROM: and RCPT TO: are responded to with a 250 which is the catch-all success message. In general, responses that start with a 2 are successes and responses that start with a 5 are errors. Common errors for these are 501 - syntax error, 503 - duplicate command, 550 - user unkown. Other errors may be server specific.
The DATA command is not a complete command, since the SMTP server must await further data. It responds with a 354 signifying that it is awaiting further data. On receipt of this data (when it receives a single . on a line by itself), it responds with a 250 on success, or a 5xx on failure.
The QUIT command signs off from the SMTP server, and it responds with a 221.
Following is the complete list of SMTP replies ordered by functional group:
Finally, I'll list a sample SMTP session. I'll use < to mark lines that the server sends back.
$telnet localhost 25 < 220 localhost ESMTP Sendmail 8.9.3/8.9.1 HELO localhost < 250 localhost Hello localhost [127.0.0.1], pleased to meet you MAIL FROM: philip < 250 philip... Sender ok RCPT TO: philip < 250 philip... Recipient ok DATA < 354 Enter mail, end with "." on a line by itself Checking SMTP transactions. . < 250 SAA11703 Message accepted for delivery QUIT < 221 localhost closing connection
I hope that this little primer will help people understand how SMTP works.
Philip