\chapter{\unix\ \MTA{}s}

This chapter introduces a group of \mta{}s.
The selected group will be delimited against other groups of \MTA{}s, which are described as well.

The chosen programs will be presented to the reader in a short overview and with the most important facts.
The next chapter will show a comparison of these programs in several disciplines.


\section{Types of \MTA{}s}
``Mail transfer agent'' is a term covering a variety of programs.
One thing is common to them: they transport email from one \emph{thing} to another.
These \emph{things} can be hosts, meaning independent machines, or protocols like \NAME{SMTP} and \NAME{UUCP}, between which mail is transfered.\footnote{\sendmail{}'s initial purpose was moving mail between \NAME{UUCP}, \name{Berknet} and \NAME{SMTP}.}

Beside this common property, \MTA{}s can be very different.
Some of them have \NAME{POP3} and/or \NAME{IMAP} servers included.
Some can fetch mails through these protocols.
Others have have every feature you can think of.
And maybe there are some that do nothing else, but transporting email.

Here are groups of \mta{}s that will not be regarded in the following chapter.
% Prof: briefly discuss the border between small MTAs (sendmail, masqmail) and state-of-the-art mailservers (postfix) and groupware (notes, exchange): give usecases.

\subsection{Relay-only \MTA{}s}
This is the most simple kind of \MTA. They transfer mail only to defined \name{smart hosts}\footnote{\name{smart host}s are \MTA{}s that receives email and route it to the actual destination}. \name{Relay-only} \MTA{}s do not receive mail, they only send it.

Most \MTA{}s can be configured to act as such a forwarder. But this is usually an additional functionality.

One would use such a program to give a system the possibility to send mail, without the need to do lots of configuration. In a local network, usually the clients are set up with such a forwarder, while there is one \name{mail server} that acts as a \name{smart host}. The ``dumb'' clients send mail only to this one \name{mail server} which does the ``real work''.

Examples for that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.


\subsection{Groupware}
Normally the term ``groupware'' does not mean one single program, but a suite of programs. They are build up a framework which can be populated with various modules that provide actual funktionality. These modules including mail transfer, file storage, calendars, resource management, instant messaging and more.

One would use one of these program suites if the main work to do is not mail transfer, but providing communication and team working support for a group of people. The most common scenario are companies. They have \name{groupware} running to provide adequate services for their teams to work efficently. But home servers may provide \name{groupware} services for the family members also.

Examples for this kind of programs are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.


\subsection{More limitations}
Third, only \emph{sendmail-compatible} \MTA{}s will be regarded in the competition.
Hence all \MTA{}s not having a \emph{sendmail-compatible} interface or not offering it as a compatibility addon, will not be covered here. %FIXME: give an example for non-sendmail-compatible
The reason is the heavy reliance of \unix\ systems on the existance of a programs called \path{sendmail}.
This has historical reasons. See section \ref{sec:sendmail} for further information.

Further more, only programs with source code available are regarded. %FIXME: why


\subsection{The ones not regarded here}
The candidates for the competition in the next chapter are a subset of the \MTA{}s available.
Comparision between totally different programs (apart of one function) makes not much sense. %FIXME: better words
One would not use a program for a job it is not suited for.
Therefor \mta{}s that are rarely similar to \masqmail\ are not regarded.


\section{The programs regarded}
The programs remaining are \emph{sendmail-compatible} ``smart'' \MTA{}s that do not offer masses of features unrelated to mail transport.

For the comparision, five programs of this group are taken.
These are: \sendmail, \name{qmail}, \name{postfix}, \name{exim} and \masqmail.
The four alternatives to \masqmail\ are the most important representatives of the regarded group.
%TODO: what about having one program as ``outsider'' ...?

Other, but not covered, group members are: %FIXME:  are these all MTAs of that group? why these and not others?
%TODO: what about `courier-mta'?

Here follows a small introduction to each of the five.

\subsection{\sendmail}
\sendmail\ is the most popular \mta.
Since it was one of the first \MTA{}s and was shipped by many vendors of \unix\ systems.

The program was written by Eric Allman as the successor of his program \name{delivermail}.
\sendmail\ was first released with \NAME{BSD} 4.1c in 1983.
Allman was not the only one working on the program.
Other people developed own versions of it and a variety of flavors came up, especially in the late eighties when Allman was inactive.

\sendmail\ is focused on transfering mails between different protocols and networks, this lead to a very flexible (though complex) configuration.

The latest version is 8.14.3 from May 2008.
The program is distributed under the \name{Sendmail License} as both, \freesw\ and proprietary software of \name{Sendmail, Inc.}.

Further development will go into the project \name{MeTA1} which succeeds \sendmail.

More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and on \citeweb{wikipedia:sendmail} and \citeweb{jdebp}.


\subsection{\name{qmail}}
\name{qmail} is seen by its community as ``a modern SMTP server which makes sendmail obsolete''.
It was written by Daniel~J.\ Bernstein starting in 1995.
His primary goal was to create a secure \MTA\ to replace the popular, but vulnerable, \sendmail.

\name{qmail} first introduced may innovative concepts in \mta\ design and is generally seen as the first security-aware \MTA\ developed.

Since November 2007, \name{qmail} is released in the \name{public domain} which makes it \freesw.
The latest release is 1.03 from July 1998.

The programs homepages are \citeweb{qmail:homepage1} and \citeweb{qmail:homepage2}. Further information about \name{qmail} is available on \citeweb{lifewithqmail}, \citeweb{wikipedia:qmail} and \citeweb{jdebp}.


\subsection{\name{postfix}}
The \name{postfix} project was started in 1999 at \name{IBM research}, then called \name{VMailer} or \name{IBM Secure Mailer}.
Wietse Venema's program ``attempts to be fast, easy to administer, and secure. The outside has a definite Sendmail-ish flavor, but the inside is completely different.''\citeweb{postfix:homepage}
In fact, \name{postfix} was mainly designed after qmail's architecture to gain security.
But in contrast to \name{qmail} it aims much more on being fast and full-featured.

Today \name{postfix} is taken by many \unix systems and \gnulinux distributions as default \MTA.

The latest stable version is numbered 2.5.5 from August 2008.
\name{postfix} is covered by the \name{IBM Public License 1.0} which is a \freesw\ license.

Additional information is available on the program's homepage \citeweb{postfix:homepage}, on \citeweb{jdebp} and \citeweb{wikipedia:postfix}.


\subsection{\name{exim}}
\name{exim} was started in 1995 by Philip Hazel at the \name{University of Cambridge}.
Its age is about the same as \name{qmail}'s, but the architecture is totally different.

While \name{qmail} took a completely new approach, \name{exim} forked of \name{smail-3}, and therefor is monolitic like that and like \sendmail.
But having no separation of the individual components of the system, like \name{qmail} and \name{postfix} have, did not hurt.
Its security is comparably good.

\name{exim} is highly configurable, especially in the field of mail policies.
This makes it easy to specify how mail is routed through the system and who is allowed to send email to whom.
Also interfaces for integration of virus and spam check programs are provided by design.

The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007.

One finds \name{exim} on its homepage \citeweb{exim:homepage}. More information about it can be retrieved from \citeweb{wikipedia:exim} and \citeweb{jdebp}.


\subsection{\masqmail}
The \masqmail\ program was written by Oliver Kurth, starting in 1999.
His aim was to create a small \mta\ which is especially focused on computers with dial-up connections to the internet.
\masqmail\ is easy configurable for situations which are rarely solveable with the common \MTA{}s.

\masqmail\ queues mail for destinations outside the local network if no connection to the internet is online.
If the machine goes online, this mail is sent.
Mail to local machines is sent immediately.

While the other \MTA{}s are more general purpose \MTA{}s, \masqmail\ aims on special situations only.
Nevertheless can it handle ordinary mail transfers too.

\masqmail\ is released under the \GPL, which makes it \freesw. The latest stable version is 0.2.21 from November 2005.

The program's new homepage \citeweb{masqmail:homepage} provides further information about this \MTA.
