\chapter{Mail transfer agents}

After having analyzed the market for electronic mail and trends for it in the last chapter, this chapter takes a look at \mta{}s, the intelligent most important parts of the email infrastructure. \MTA{}s will be grouped by similarities, and the four most popular \freesw\ \mta{}s, will be presented to the reader in a short overview and with the most important facts. At the end of this chapter a comparison of these programs in several disciplines will be done.




\section{Types of MTAs}
``Mail transfer agent'' is a term covering a variety of programs. One thing is common to them: they transfer email from one machine to another.

This is how \person{Bryan Costales} defines a \mta\ in \cite{costales97}:
\begin{quote}
A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office.
\end{quote}
\name{The Free Dictionary} is a bit more concrete on the term: \citeweb{website:thefreedictionary}
\begin{quote}
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.
\end{quote}

Common to all \MTA{}s is the transfer of mail to other machines; this is the actual job. Besides this similarity, \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 all features you can think of. And maybe there are some that do nothing else but transporting email.

Following is a classification of \mta{}s into groups of similar programs, regarding what is viewable from the outside.


\subsubsection*{Relay-only MTAs}
\label{subsec:relay-only}
This is the most simple kind of \MTA. It transfers 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 from outside the system, and they do not deliver locally.

Most \MTA{}s can be configured to act as such a \name{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 a \name{relay-only} \MTA, while there is one \name{mail server} that acts as a \name{smart host}. The ``dumb'' clients send mail to this one \name{mail server} which does all the work.

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


\subsubsection*{Groupware}
Normally the term ``groupware'' does not mean one single program, but a suite of programs. They build a framework which is then populated with various modules that provide actual functionality. Modules for mail transfer, file storage, calendars, resource management, instant messaging, etc., are commonly available.

One would use one of these program suites if the main work to do is not mail transfer, but providing integrated communication facilities 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 efficiently. But one may use \name{groupware} on the home server for his family members also.

Examples are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.


\subsubsection*{``Real'' MTAs}
There is a third type of \mta{}s in between the minimalistic \name{relay-only} \MTA{}s and the bloated \name{groupware}. Those programs may be named ``real \MTA{}s'', or ``proper \MTA{}s'', though there is no common name. They are what is meant with the term ``\mta''---programs that transfer mail between hosts.

Common to them is their focus on transferring email, while being able to act as \name{smart host}. Their variety ranges from ones mostly restricted to mail transfer (\qmail) to others already having interfaces for adding further mail processing modules (\postfix). They cover everything in between the other two groups.  %FIXME: are postfix and qmail good examples?

This group is of importance in this document. All programs selected for the comparison in the following section are ``real \MTA{}s''. \masqmail\ is one too.


\subsubsection*{Other segmenting}
\name{Mail transfer agents} can also be split in other ways.
\begin{itemize}
\item
Due to \sendmail's significance---described in section \ref{sec:sendmail}---compatibility interfaces for \sendmail\ are of importance for \unix\ \MTA{}s. Being not \emph{sendmail-compatible} does not need to matter for some fields of action, but makes the program ineligible for serving as a general purpose \MTA\ on \unix\ systems. Hence being sendmail-compatible is a major property of a \mta. %todo: how many MTAs are sendmail-compatible?
\MTA{}s not having a \emph{sendmail-compatible} interface or not offering it as a compatibility add-on, will not be covered here. One example for such a program is \name{Apache James}.  %FIXME: check if correct

\item
Another separation can be done between \freesw\ programs and proprietary software. Many of the \MTA{}s for \unix\ systems are \freesw. Only these are regarded in the following sections, because comparing \freesw\ with proprietary or commercial software is not what typical users of programs like \masqmail\ do. %fixme: what are typical users?
Comparison with those non-free programs may be a point for large \freesw\ projects, trying to step into the business world. Small projects, mostly used by individuals at home, %fixme: is this the right target field? see chap02
need to be compared against other projects of similar shape. The document should be seen from \masqmail's point of view---an \MTA\ for a \unix\ system on home servers, workstations, or maybe embedded platforms---so non-free software is out of the way.
\end{itemize}




\section{Popular MTAs}

This section introduces a selection of popular \MTA{}s; they are the most likely substitutes for \masqmail. All are \emph{sendmail-compatible} ``smart'' \freesw\ \MTA{}s that focus on mail transfer, as is \masqmail.

The programs chosen are: \sendmail, \exim, \qmail, and \postfix. They are the most important representatives of the regarded group. Although \MTA\ statistics are rare, FIXME(have different results), and good data is hard to collect, these programs tend to stay near the top.

Table \ref{tab:mta-market-share} shows the Top 10 \MTA{}s of three different statistics. The first published by \name{O'ReillyNet} in 2007 \citeweb{oreillynet:mta-stats} , the second by \name{MailRadar.com} from YYYY \citeweb{mailradar:mta-stats} , and the third by \person{Daniel~J.\ Bernstein} (the author of \qmail) done in 2001 \citeweb{djb:mta-stats}.

\begin{table}
	\begin{center}
		\input{input/mta-market-share.tex}
	\end{center}
	\caption{Market share of \MTA{}s}
	\label{tab:mta-market-share}
\end{table}

Other members of the same group are: \name{smail}, \name{zmailer}, \name{MMDF}, and \name{courier-mta}. They all are less important and rarely used, thus ommited here.


Now follows a small introduction to the five programs chosen for comparison, except \masqmail\ which already was introduced in chapter \ref{chap:introduction}. Longer introductions, including analysis and comparison, were written by \person{Jonathan de Boyne Pollard} \citeweb{jdebp}.



\subsubsection*{sendmail}
\label{sec:sendmail}
\sendmail\ is the most popular \mta, since it was one of the first and was shipped as default \MTA{}s by many vendors of \unix\ systems. %fixme: ref

The program was written by \person{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. %fixme: ref

\sendmail\ is focused on transferring 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} (the former name was \name{sendmail X}) which succeeds \sendmail.

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



\subsubsection*{exim}
\label{sec:exim}
\exim\ was started in 1995 by \person{Philip Hazel} at the \name{University of Cambridge}. It is forked of \name{smail-3}, and inherited the monolithic architecture, similar to \sendmail's. But having no separation of the individual components of the system, like \qmail\ and \postfix\ have, did not hurt. Its security is comparably good. %fixme: ref

\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. %fixme: ref

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

One finds \exim\ on its homepage \citeweb{exim:homepage}.



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

\qmail\ first introduced many innovative concepts in \mta\ design and is generally seen as the first security-aware \MTA\ developed. %fixme:ref
%fixme: what about mmdf?

Since November 2007, \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 \qmail\ is available with \person{Dave Sill}'s ``Life with qmail'' \citeweb{lifewithqmail}.



\subsubsection*{postfix}
\label{sec:postfix}
The \postfix\ project was started in 1999 at \name{IBM research}, then called \name{VMailer} or \name{IBM Secure Mailer}. \person{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, \postfix\ was mainly designed after qmail's architecture to gain security. But in contrast to \qmail\ it aims much more on being fast and full-featured.

Today \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. \postfix\ is covered by the \name{IBM Public License 1.0} which is a \freesw\ license.

Additional information can be retrieved from the program's homepage \citeweb{postfix:homepage}.






\section{Comparison of MTAs}

This section does not try to provide an overall \MTA\ comparison, because this is already done by others. Remarkable comparisons are the one by \person{Dan Shearer} \cite{shearer06} and a discussion on the mailing list \name{plug@lists.q-linux.com} \citeweb{plug:mtas}. Tabulary overviews may be found at \citeweb{mailsoftware42}, \citeweb{wikipedia:comparison-of-mail-servers}, and \citeweb[section 1.9]{lifewithqmail}.

Here provided is an overview on a selection of important properties, covering the four previously introduced programs. The data comes from the above stated sources and is collected in table \ref{tab:mta-comparison}.

\begin{table}
	\begin{center}
		\input{input/mta-comparison.tex}
	\end{center}
	\caption{Comparison of MTAs}
	\label{tab:mta-comparison}
\end{table}


\subsection{Architecture}

Architecture is most important when comparing \MTA{}s. Many other properties of a program depend on its architecture. %fixme: add ref?
\person{Munawar Hafiz} \cite{hafiz05} discusses in detail on \mta\ architecture, comparing \sendmail, \qmail, \postfix, and \name{sendmail X}. \person{Jonathan de Boyne Pollard}'s \MTA\ review \citeweb{jdebp} is a source too.

Two different architecture types show off: monolithic and modular \mta{}s.

Monolithic \MTA{}s are \sendmail, \name{smail}, \exim, and \masqmail. They all consist of one single \emph{setuid root}\footnote{\emph{setuid root} lets a program run with the rights of its owner, here root. This is considered a security risk.} binary which does all the work.

Modular \MTA{}s are \NAME{MMDF}, \qmail, \postfix, and \name{MeTA1}. They consist of several programs, each doing a part of the overall job. The different programs run with the least permissions the need, and \emph{setuid root} needs not to be used.

The architecture does not directly define the program's security, but ``[t]he goal of making a software secure can be better achieved by making the design simple and easier to understand and verify''\cite[chapter 6]{hafiz05}. \exim, though being monolithic, has a fairly clean security record. But it is very hard to keep the security up, as the program growth. \person{Wietse Venema} (the author of \postfix) says, the architecture enabled \postfix\ to grow without running into security problems. \citeweb[page 13]{venema:postfix-growth}

The modular design, with each sub-program doing one part of the overall job, is applied \name{Unix Philosophy}. The Unix Philosophy \cite{gancarz95} demands ``small is beautiful'' and ``make each program do one thing well''. Monolithic \MTA{}s fail here.

Today modular \mta\ architectures are the state-of-the-art.



\subsection{With focus on the future}

Section \ref{sec:what-will-be-important} tried to figure out the importances for future \MTA{}s. The four programs are compared on these (possible) future requirements now.

The first trend was provider independence, requiring easy configuration. \postfix\ seems to do best here. It used primary two configuration files (\path{master.cf} and \path{main.cf}) which are easy to manage. \sendmail\ appears to have a bad position. Its configuration file \path{sendmail.cf} is very complex, including Turing-completeness, thus it needs simplification wrappers around it to provide easier configuration. There exist the \name{m4} macros to generate \path{sendmail.cf}, but adjusting the generated result by hand seems to be nessesary for non-trivial configurations. \qmail's configuration files are simple, but the whole system is complex to set up; it requires various system users and is hardly usable without applying several patches to add basic functionality. \name{netqmail} is the community effort to help here. \exim\ has only one single configuration file (\path{exim.conf}), but it suffers most from its flexibility, like \sendmail. Flexibility and easy configuration are contrary.

As second trend, the decreasing nessesarity for high performance was identified. This goes along with the move of \MTA{}s from service providers to home servers. \postfix\ focuses much on performance, this might not be an important point then. Of course there still will be the need for high performance \MTA{}s, but a growing share of the market will not require high performance. Performance is related to simplicity, which effects security. Increasing performance does in most times decrease the other two. Simple \mta{}s not aiming for highest performance are what is needed in future. The simple of \qmail, still being fast enough, seems to be a good example.

The third trend, even more security awareness, is addressed by each of the four programs. It seems as if all widely used \mta{}s provide good security nowadays---even \sendmail\ can be considered secure today. %fixme:ref
The modular architecture, used by \qmail\ and \postfix, is generally seen to be conceptually more secure, however.%fixme: ref
\sendmail's creators have started \name{MeTA1}, a modular \MTA\ merging the best of \qmail\ and \postfix, to replace the old \sendmail. It will be interesting to watch \exim's future---will it become modular too?







%todo: my own poll (?)


%<< complexity >> << security >> << simplicity of configuration and administration >> << flexibility of configuration and administration >> << code size >> << code quality >> << documentation (amount and quality) >> << community (amount and quality) >> << used it myself >> << had problems with it >>


%<< quality criteria >> << standards of any kind >> << how to compare? >> << (bewertungsmatrix) objectivity >> << how many criteria for ``good''? >>

