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

This chapter introduces a group of \mta{}s.
These will be presented to the reader in a short overview and with the most important facts.
The next chapter will show a comparison of the candidates 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.


\section{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.
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.

The first group of programs to sort out are the so called \name{groupware} programs.
These provide a whole lot of functions, including mail transfer, file storage, calendars, resource management, instant messaging and more.
Examples for this kind of programs are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.

The second group are the \name{relay-only} \MTA{}s.
They transfer mail only to defined \name{smart hosts}\index{smart host}\footnote{\MTA{}s that receives email and route it to the actual destination}.
Most \MTA{}s can be configured to act as such a forwarder.
But this is normally an additional functionality.
Examples for that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.

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.
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


\section{The competitors}
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'?

% http://www.yolinux.com/TUTORIALS/LinuxTutorialMailMTA.html
% http://homepages.tesco.net/~J.deBoynePollard/Reviews/UnixMTSes/

Here follows a small introduction to each of the five.

\subsection{\sendmail}
%TODO: author, date of first release, basic intention of program, ...
%TODO: references to various descriptions
%\citeweb{homepage:sendmail},
\citeweb{wikipedia:sendmail}, \citeweb{jdebp}

\subsection{\name{qmail}}
%TODO: author, date of first release, basic intention of program, ...
%TODO: references to various descriptions

\subsection{\name{postfix}}
%TODO: author, date of first release, basic intention of program, ...
%TODO: references to various descriptions

\subsection{\name{exim}}
%TODO: author, date of first release, basic intention of program, ...
%TODO: references to various descriptions

\subsection{\masqmail}
The \masqmail\ program was written by Oliver Kurth, starting in 1999.
His aim was to create a \mta\ which is especially focused on computers with dial-up connections to the internet.
\masqmail\ handles situations which are rarely solveable with the common \MTA{}s.
%TODO: references to various descriptions


% briefly discuss the border between small MTAs (sendmail, masqmail) and state-of-the-art mailservers (postfix) and groupware (notes, exchange): give usecases.
