Brief Review of E-mail Concepts

Transferring e-mail between users on a single machine is a relatively trivial process. However, transporting e-mail between a wide-array of TCP/IP hosts and servers globally across the Internet can be quite complex. The protocol used to transport mail across the Internet is the SMTP [3], but the actual transport management of global e-mail requires a much more complex electronic infrastructure.

Servers and programs with a primary function to store and forward e-mail across the vast canyons of cyberspace are referred to as MTAs. A mail transfer agent (MTA) is a very specialized program which delivers and transports e-mail between mail servers. On the other hand, a mail user agent (MUA) may be any one of a vast number of programs users execute to read, compose, reply, and manage e-mail locally. The widely used sendmail program is an MTA [4].

SMTP messages consist of lines of ASCII text of information in a rigid format followed by the main body of the message. The rigidly-formated section is known as the header. RFC 822 [5] defines the syntax and specification of the SMTP header, created to allow simple parsers to process the general structure of messages without knowledge of the detailed structure of individual header fields.

The sendmail MTA was designed to manage a very complex internetworking environment and is one of the cornerstones of the Internet [4]. One of the myriad technical requirements for sendmail was the necessity to process addresses that are in route address syntax [4] [6]. These addresses follow a syntax created for directed source message routing:

@MTA₁,@MTA₂,@MTA_n:USER@MTA_n+1

In route address syntax, this expression translates to:

1.

the originator sends the message to MTA₁,

2.

MTA₁ sends the message to MTA₂,

3.

MTA₂ sends the message to MTA_n,

4.

MTA_n sends the message to MTA_n+1,

5.

MTA_n+1 delivers the message to USER.

The capability of sendmail to process route addressing is but one of numerous uncountable special rules and expressions which a robust universal MTA must manage. The next section discusses how this syntax is exploited to create extraordinary e-mail bombs. The reader is kindly referred to Brian Costales and Eric Allman excellent book on sendmail [4] for further reading on the sendmail MTA and its capabilities.