The process of exchanging email is based upon a number of different protocols, but at the heart of electronic email is the simple mail transport protocol (SMTP).

This protocol is based upon the connection-orientated TCP protocol and is certainly the most widely used email transport in the Internet today. When an email is sent between MTA's, SMTP is the protocol that these email servers "speak". SMTP is situated at the top of the TCP/IP stack - pretty much on top of TCP.

We will spend some time later conducting a conversation with an SMTP server. For now though, we have a little more groundwork to cover.

When I send a mail to joe@mailprompt.co.za, my MUA delivers the email to my local SMTP server.

The SMTP server (let 's call it smtp.QEDux.co.za) resolves the IP address of the mail server for mailprompt.co.za and attempts to contact that server on port 25 - which is the default SMTP port.

Note

The SMTP server is able to resolve the IP address for the mailprompt.co.za mailer using a special DNS record - the MX record in the DNS server for mailprompt.co.za. Although this point is out of the scope of this course, it is at least nice to know how one SMTP server is able to just "know" who the mail server is on the other side. In the DNS elective course you will learn how to use "dig" command to determine IP addresses for specific URL's, including how to use the MX records to determine the IP addresses of MTA's.

Once my SMTP server has made contact with mail.mailprompt.co.za (I'll assume this is the name of the mailprompt.co.za email server), it will begin transmitting the message to the remote side.

Only once the email has been completely and successfully sent, will the local MTA (my side) delete this email from the list of email's to be sent. Once deleted, my mailer marks this email as "delivered" while the receiving SMTP server has the responsibility of ensuring the mail is delivered to the recipient.

(Delivery is a topic that is dealt with later in this module.)

Since this is essentially a point-to-point service, it is highly unlikely that email can get lost en-route. Obviously mail does sometimes get mislaid, which proves that even with this "fail-safe" mechanism, snags can still plague it.

So, what does the 'conversation' between the two mail servers look like? I have included a sample conversation below.

Simply put, there are a number of commands a SMTP server understands. In this session, the commands HELO, MAIL FROM, RCPT TO, DATA and "." are all SMTP commands:

telnet smtp.uninetwork.co.za 25
Trying 168.210.56.254...
Connected to smtp.uninetwork.co.za.
Escape character is '^]'.
220 mail.uninetwork.co.za ESMTP
HELO QEDux.co.za
250 mail.uninetwork.co.za
MAIL FROM: hamish@QEDux.co.za
250 OK
RCPT TO: andrexxxxxxx@richemont.com
250 OK
DATA
354 go ahead
This is a test email Andre,
Please discard
Cheers
Hamish
.
250 OK 1079988036 qp 12140
                

Here I have simulated a simple conversation. You can do the same on your mail server, and you should achieve a similar result.

This same conversation happens for each piece of email routed through the Internet.