
meillo@31: \chapter{\unix\ \MTA{}s}
meillo@27: 
meillo@44: This chapter introduces a group of \mta{}s.
meillo@64: The selected group will be delimited against other groups of \MTA{}s, which are described as well.
meillo@64: 
meillo@64: The chosen programs will be presented to the reader in a short overview and with the most important facts.
meillo@64: The next chapter will show a comparison of these programs in several disciplines.
meillo@28: 
meillo@27: 
meillo@28: \section{Types of \MTA{}s}
meillo@44: ``Mail transfer agent'' is a term covering a variety of programs.
meillo@44: One thing is common to them: they transport email from one \emph{thing} to another.
meillo@44: 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}.}
meillo@28: 
meillo@44: Beside this common property, \MTA{}s can be very different.
meillo@44: Some of them have \NAME{POP3} and/or \NAME{IMAP} servers included.
meillo@44: Some can fetch mails through these protocols.
meillo@44: Others have have every feature you can think of.
meillo@44: And maybe there are some that do nothing else, but transporting email.
meillo@28: 
meillo@64: Here are groups of \mta{}s that will not be regarded in the following chapter.
meillo@64: % Prof: briefly discuss the border between small MTAs (sendmail, masqmail) and state-of-the-art mailservers (postfix) and groupware (notes, exchange): give usecases.
meillo@31: 
meillo@64: \subsection{Relay-only \MTA{}s}
meillo@64: 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.
meillo@64: 
meillo@64: Most \MTA{}s can be configured to act as such a forwarder. But this is usually an additional functionality.
meillo@64: 
meillo@64: 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''.
meillo@27: 
meillo@64: Examples for that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.
meillo@64: 
meillo@64: 
meillo@64: \subsection{Groupware}
meillo@64: 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.
meillo@64: 
meillo@64: 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.
meillo@64: 
meillo@40: Examples for this kind of programs are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.
meillo@27: 
meillo@28: 
meillo@64: \subsection{More limitations}
meillo@44: Third, only \emph{sendmail-compatible} \MTA{}s will be regarded in the competition.
meillo@64: 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
meillo@44: The reason is the heavy reliance of \unix\ systems on the existance of a programs called \path{sendmail}.
meillo@44: This has historical reasons. See section \ref{sec:sendmail} for further information.
meillo@44: 
meillo@44: Further more, only programs with source code available are regarded. %FIXME: why
meillo@27: 
meillo@27: 
meillo@64: \subsection{The ones not regarded here}
meillo@64: The candidates for the competition in the next chapter are a subset of the \MTA{}s available.
meillo@64: Comparision between totally different programs (apart of one function) makes not much sense. %FIXME: better words
meillo@64: One would not use a program for a job it is not suited for.
meillo@64: Therefor \mta{}s that are rarely similar to \masqmail\ are not regarded.
meillo@64: 
meillo@64: 
meillo@64: \section{The programs regarded}
meillo@28: The programs remaining are \emph{sendmail-compatible} ``smart'' \MTA{}s that do not offer masses of features unrelated to mail transport.
meillo@27: 
meillo@44: For the comparision, five programs of this group are taken.
meillo@44: These are: \sendmail, \name{qmail}, \name{postfix}, \name{exim} and \masqmail.
meillo@44: The four alternatives to \masqmail\ are the most important representatives of the regarded group.
meillo@44: %TODO: what about having one program as ``outsider'' ...?
meillo@44: 
meillo@44: Other, but not covered, group members are: %FIXME:  are these all MTAs of that group? why these and not others?
meillo@58: %TODO: what about `courier-mta'?
meillo@27: 
meillo@28: Here follows a small introduction to each of the five.
meillo@27: 
meillo@28: \subsection{\sendmail}
meillo@60: \sendmail\ is the most popular \mta.
meillo@60: Since it was one of the first \MTA{}s and was shipped by many vendors of \unix\ systems.
meillo@60: 
meillo@60: The program was written by Eric Allman as the successor of his program \name{delivermail}.
meillo@60: \sendmail\ was first released with \NAME{BSD} 4.1c in 1983.
meillo@60: Allman was not the only one working on the program.
meillo@60: Other people developed own versions of it and a variety of flavors came up, especially in the late eighties when Allman was inactive.
meillo@60: 
meillo@60: \sendmail\ is focused on transfering mails between different protocols and networks, this lead to a very flexible (though complex) configuration.
meillo@60: 
meillo@60: The latest version is 8.14.3 from May 2008.
meillo@64: The program is distributed under the \name{Sendmail License} as both, \freesw\ and proprietary software of \name{Sendmail, Inc.}.
meillo@64: 
meillo@60: Further development will go into the project \name{MeTA1} which succeeds \sendmail.
meillo@60: 
meillo@60: More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and on \citeweb{wikipedia:sendmail} and \citeweb{jdebp}.
meillo@60: 
meillo@28: 
meillo@28: \subsection{\name{qmail}}
meillo@60: \name{qmail} is seen by its community as ``a modern SMTP server which makes sendmail obsolete''.
meillo@60: It was written by Daniel~J.\ Bernstein starting in 1995.
meillo@60: His primary goal was to create a secure \MTA\ to replace the popular, but vulnerable, \sendmail.
meillo@60: 
meillo@60: \name{qmail} first introduced may innovative concepts in \mta\ design and is generally seen as the first security-aware \MTA\ developed.
meillo@60: 
meillo@60: Since November 2007, \name{qmail} is released in the \name{public domain} which makes it \freesw.
meillo@60: The latest release is 1.03 from July 1998.
meillo@60: 
meillo@60: 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}.
meillo@60: 
meillo@27: 
meillo@28: \subsection{\name{postfix}}
meillo@64: The \name{postfix} project was started in 1999 at \name{IBM research}, then called \name{VMailer} or \name{IBM Secure Mailer}.
meillo@64: 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}
meillo@64: In fact, \name{postfix} was mainly designed after qmail's architecture to gain security.
meillo@64: But in contrast to \name{qmail} it aims much more on being fast and full-featured.
meillo@64: 
meillo@64: Today \name{postfix} is taken by many \unix systems and \gnulinux distributions as default \MTA.
meillo@64: 
meillo@64: The latest stable version is numbered 2.5.5 from August 2008.
meillo@64: \name{postfix} is covered by the \name{IBM Public License 1.0} which is a \freesw\ license.
meillo@64: 
meillo@64: Additional information is available on the program's homepage \citeweb{postfix:homepage}, on \citeweb{jdebp} and \citeweb{wikipedia:postfix}.
meillo@64: 
meillo@27: 
meillo@28: \subsection{\name{exim}}
meillo@64: \name{exim} was started in 1995 by Philip Hazel at the \name{University of Cambridge}.
meillo@64: Its age is about the same as \name{qmail}'s, but the architecture is totally different.
meillo@64: 
meillo@64: While \name{qmail} took a completely new approach, \name{exim} forked of \name{smail-3}, and therefor is monolitic like that and like \sendmail.
meillo@64: But having no separation of the individual components of the system, like \name{qmail} and \name{postfix} have, did not hurt.
meillo@64: Its security is comparably good.
meillo@64: 
meillo@64: \name{exim} is highly configurable, especially in the field of mail policies.
meillo@64: This makes it easy to specify how mail is routed through the system and who is allowed to send email to whom.
meillo@64: Also interfaces for integration of virus and spam check programs are provided by design.
meillo@64: 
meillo@64: The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007.
meillo@64: 
meillo@64: One finds \name{exim} on its homepage \citeweb{exim:homepage}. More information about it can be retrieved from \citeweb{wikipedia:exim} and \citeweb{jdebp}.
meillo@64: 
meillo@28: 
meillo@28: \subsection{\masqmail}
meillo@44: The \masqmail\ program was written by Oliver Kurth, starting in 1999.
meillo@64: His aim was to create a small \mta\ which is especially focused on computers with dial-up connections to the internet.
meillo@64: \masqmail\ is easy configurable for situations which are rarely solveable with the common \MTA{}s.
meillo@64: 
meillo@64: \masqmail\ queues mail for destinations outside the local network if no connection to the internet is online.
meillo@64: If the machine goes online, this mail is sent.
meillo@64: Mail to local machines is sent immediately.
meillo@27: 
meillo@64: While the other \MTA{}s are more general purpose \MTA{}s, \masqmail\ aims on special situations only.
meillo@64: Nevertheless can it handle ordinary mail transfers too.
meillo@27: 
meillo@64: \masqmail\ is released under the \GPL, which makes it \freesw. The latest stable version is 0.2.21 from November 2005.
meillo@64: 
meillo@64: The program's new homepage \citeweb{masqmail:homepage} provides further information about this \MTA.




