\chapter{Introduction}
\label{chap:introduction}

\section{History of electronic mail}
%TODO: have a quote from Bell Labs about email here

Electronic mail\index{electronic mail} (short: \name{email})\citeweb{wikipedia:email} is a basic concept in \unix.\citeweb{unix-mail-intro} On \unix\ machines, a lot of information is distributed by \name{system mail}, which is email sent by the operating system. Beside that, email is the common communication system between humans working on computers.

The \unix\ operating system supports email through the \name{mail user agent} (short: \NAME{MUA}) \name{/bin/mail}.

Development of \unix\ was not only made in the \name{Bell Labratories} of \NAME{AT\&T}. The \name{Univerity of California at Berkeley} worked on their version of a \unix\ operating system, too. It is refered to as \NAME{UCB} \unix, or \name{Berkeley} \unix\index{Berkeley Unix}.

The few features of \name{/bin/mail} lead to a second \NAME{MUA} from Berkeley: \name{Mail} (with a capital `M'). Later, the superior functionality of \name{Mail} went back to \name{Bell Labs} and into the program \name{mailx}, the successor of \name{/bin/mail}. %The different programs for handling mail are a good example for the problem of standardization of \unix. %TODO: need that last sentence?

Nowadays, \name{mailx} and \name{Mail} are quite equivalent and \name{/bin/mail} is linked to either of them---whichever is installed.

At that time, computers were connected by various kinds of networks. \name{Bell Labs} had invented the \NAME{UUCP} program and protocol suite (for ``\unix\ to \unix\ copy'')\citeweb{wikipedia:uucp}. Berkeley however had an own creation called \name{Berknet} in use. And the \name{United States Department of Defence Advanded Research Projects Agency}'s (\NAME{ARPA}) effort on designing a new wide area network, led to the \NAME{ARPANET}\citeweb{wikipedia:arpanet}, based on the \name{transmission control protocol} (\NAME{TCP}). There were also other, minor, kinds of networks in use.

Email was transfered between different machines within the same networks. The file transfer itself was made uniformly using \NAME{FTP}, but the higher layered logic of the transfer was different. For example was addressing done different: \NAME{UUCP} used a flat-style schema, while \NAME{ARPANET}'s was hierachical.

Mail transport from one machine connected to one kind of network to a second machine connected to another was a problem. This showed up at Berkeley where some departments of the university had switched to \NAME{ARPANET}, and some to \NAME{UUCP}, while the rest used \name{Berknet}.

It was around 1982, when Eric Allman, then a student at Berkeley, wrote \name{delivermail}. Its purpose was to transform email from one network to another. \name{delivermail}, like its successor---the more flexible \sendmail---intermediated between the different networks. They were able to transform email messages from any network to any other.

Todays email structure is basicly the same as then. The major difference is the uniformity of the underlying network, which is nearly always the \NAME{ARPANET}-based \name{Internet}. Hence lowering the importance of the transformation capabilities of \MTA{}s, that was essential to \sendmail's success---yet being the primary motivation for the program.

More information about the history of electronic mail can be found at: \citeweb{email:griffiths}, \citeweb{email:crocker}, \citeweb{email:vleck}, \citeweb{email:akkad}, \citeweb{email:murakami}, and \citeweb{email:tomlinson}. A good starting point for general information on internet history is \citeweb{wikipedia:historyoftheinternet}.
%TODO: check the websites which ones are the important ones; remove unnessesary ones



\section{Transporting mail}
%TODO: when was the term ``mail transfer agent'' established?

This thesis is about a \name{mail transfer agent} (or \index{mail transport agent|see{mail transfer agent}}\name{mail transport agent}, short \NAME{MTA}): \masqmail. \sendmail\ is one too---the most important one.

The basic job of a \mta\ is to transfer/transport electronic mail from one host to another.

Here are definitions from others:

\begin{quotation}
A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office.
\cite{costales97}
\end{quotation}

\begin{quotation}
A mail transfer agent (MTA) (also called a mail transport agent, message transfer agent, or smtpd (short for SMTP daemon)), is a computer program or software agent that transfers electronic mail messages from one computer to another.
\citeweb{wikipedia:mta}
\end{quotation}

\begin{quotation}
mail server (also known as a mail transfer agent or MTA, a mail transport agent, a mail router or an Internet mailer) is an application that receives incoming e-mail from local users (people within the same domain) and remote senders and forwards outgoing e-mail for delivery.
\citeweb{website:techtarget}
\end{quotation}

\begin{quotation}
Message Transfer Agent - (MTA, Mail Transfer Agent): Any program responsible for delivering e-mail messages. Upon receiving a message from a Mail User Agent or another MTA, [...] it [...] delivers it to any local addressees and/or forwards it to other remote MTAs (routing) for delivery to remote recipients.
%Any program responsible for delivering e-mail messages. Upon receiving a message from a Mail User Agent or another MTA, often by SMTP over the Internet, it stores it temporarily locally and analyses the recipients and delivers it to any local addressees and/or forwards it to other remote MTAs (routing) for delivery to remote recipients. In either case it may edit and/or add to the message headers.
%
%The most widely used MTA for Unix is sendmail, which communicates using SMTP.
%
%RFC 2821 (SMTP) expands MTA as ``Mail Transfer Agent'' though this is less common. Alternatives with ``Transport'' are also seen but less correct.
\citeweb{website:thefreedictionary}
\end{quotation}

Common is the transfer of mail to other machines; this is the actual job. \MTA{}s work with mail, received from local users and/or remote machines. Mail delivery however is \emph{not} what \mta{}s are for, although probably every \MTA\ is able to deliver mail, and many do. \name{mail delivery agents} (short: \NAME{MDA}) are the programs for this job. Two of the best known \NAME{MDA}s are \name{procmail} and \name{maildrop}.



\section{\sendmail}
\label{sec:sendmail}

Allman wrote it to transfer emails between different networks, thus giving \sendmail\ mighty address rewriting abilities. In contrast to its predecessor \name{delivermail}, was \sendmail\ designed to offer greatest flexiblity in configuration; this enabled it to deal with any type of network.

\sendmail\ was, and still is, very successful. So successful that it stands, like no other, for the whole group of \MTA{}s: \name{sendmail} actually is the \emph{de facto standard} for \mta{}s.

Its author, Allman, sees three reasons for the huge success: the ``sloopy'' approach (accepting badly formed messages); its focus on the routing function; and the flexible configuration (this was important in \sendmail's early days).
\cite[page xviii]{costales97}

Others see \sendmail's success more critical. One of them is quoted in the \name{MMDF} FAQs \citeweb{faqs:mmdf}:
\begin{quotation}
Sendmail was once compared by one old Internet hand to ``those killer bees that escaped from the laboratory---and now they're everywhere and you can't get rid of 'em''.
\end{quotation}
He definately hints here at \sendmail's many security vulnerabilities that came to light and on its complexity, in particular its obscure configuration file \path{sendmail.cf}.

No matter how \sendmail\ is seen, one must admit its influence on \unix\ emailing programs. Most existing substitutes mimic \sendmail's interface and behavior. Most notable, they create a symbolic link named ``sendmail'' pointing to their own executable. The reason herefor are the many programs assuming an executable called ``sendmail'' on every computer system existing.

\sendmail\ is not only ported to many platforms, even including \name{Microsoft Windows}, but also it is still the prefered \MTA\ on many systems.

For deeper knowledge on \sendmail's history, see \cite{costales97} and \cite{vixie01}.


\section{(a look at Windows)}
% TODO: add content here
