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How Email Works
Technical Brief
The following information explains the terminology and technology involved with email.
SMTP Servers - Most email traffic on the Internet is delivered using the SMTP protocol through SMTP servers. I break down SMTP into two servers: Inbound and Outbound.
Inbound SMTP - Accepts connections from other SMTP servers and email clients. Its job description is to accept mail from a source address and make the email message available to the MTA.
Outbound SMTP - Connects to SMTP servers on foreign networks and delivers mail using the SMTP protocol.
MTA - An acronym for Message Transport Agent. The brains of an email server, it accepts mail from an SMTP server and routes it to local email boxes or to the Outbound SMTP server.
POP3 Server - Makes email stored for a given user available for download using the POP3 protocol.
Email Clients are comprised of two components:
POP3 Client - Connects to a POP3 server, requests authentication and downloads messages.
SMTP Client - This is really a reduced version of the Outbound SMTP server that sends mail to an SMTP server that is backed by an MTA. The protocol, however, is the same: SMTP.
SMTP Protocol - This is an open ASCII text protocol that can be implemented even by hand using any Telnet tool. While the implementation can be complex, it can also be over simplified to 3 commands:
| Command |
Description |
| MAIL FROM: |
Identifies who the mail is from. |
| RCPT TO: |
Identifies whom the mail is TO. This command is repeated for each recipient. |
| DATA: |
The bulk of the message. This contains the email headers and body. |
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Note that the email headers you see in an email message have almost nothing to do with actually sending the mail. This is why you can see bogus information in the 'From' and 'To' fields of an email header. An SMTP Server (inbound) accepts the above commands and an SMTP Outbound Server or Client sends the above commands.
POP3 Protocol - As with SMTP, this is another open ASCII protocol. It too can be boiled down to a handful of commands:
| Command |
Description |
| USER |
Username of the user. |
| PASS |
Password of the user. |
| List |
List messages on server. |
| RETR (X) |
Retrieve message number X. |
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Obviously there are more commands than these for both POP3/SMTP and the actual implementation is very complex in today's email environment.
Domain Name Servers and Clients - DNS or Domain Name Server as applied to email is a service frequently used by outbound SMTP servers to look up the IP Address of a domain. A special record in the domain name body called the 'MX' record exists to specifically deal with where an email server is.
FOR EXAMPLE - Suppose you sent an email message to your local server (bendcable.com) using your email client to someone named gregoryway@blockallspam.com. The Outbound SMTP service would get the mail from the MTA server and based on the RCPT TO command, realize that it must know the IP Address of blockallspam.com. The MTA software pulls up the blockallspam.com DNS record and finds the MX entry for the Inbound SMTP server for blockallspam.com. It would get back the information below and connect to 127.0.0.1 to send the message:
blockallspam.com MX 0 mail.blockallspam.com
mail.blockallspam.com A 127.0.0.1
DNS is also used in SPF. Please refer to our TechTalk on SPF for more information.
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