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annotate thesis/tex/3-MailTransferAgents.tex @ 261:b390fb627f10

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author meillo@marmaro.de
date Wed, 14 Jan 2009 12:49:22 +0100
parents db81f3cc6675
children 6ed93483bba9
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meillo@89 1 \chapter{Mail transfer agents}
meillo@254 2 \label{chap:mail-transfer-agents}
meillo@89 3
meillo@217 4 After having analyzed the market for electronic mail and identified upcoming trends, in the last chapter; this chapter takes a look at \mta{}s---the intelligent nodes and thus the most important parts of the email infrastructure. The \MTA{}s will be grouped by similarities first. Then 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 these programs will be compared.
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meillo@89 8
meillo@120 9 \section{Types of MTAs}
meillo@217 10 ``Mail transfer agent'' is a term covering a variety of programs. One thing is common to them: they transfer email from one senders to recipients.
meillo@89 11
meillo@248 12 This is how \person{Bryan Costales} defines a \mta:
meillo@117 13 \begin{quote}
meillo@217 14 A mail transfer agent (\MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office.
meillo@218 15 \hfill\cite{costales97}
meillo@117 16 \end{quote}
meillo@217 17 \name{The Free Dictionary} is a bit more concrete on the term:
meillo@117 18 \begin{quote}
meillo@217 19 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 \MTA{}s (routing) for delivery to remote recipients.
meillo@218 20 \hfill\citeweb{website:thefreedictionary}
meillo@117 21 \end{quote}
meillo@89 22
meillo@259 23 \person{Dent} and \person{Hafiz} agree \cite[page 19]{dent04} \cite[pages 3-5]{hafiz05}.
meillo@259 24
meillo@259 25 Common to all \MTA{}s is the transport of mail; 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.
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meillo@117 27 Following is a classification of \mta{}s into groups of similar programs, regarding what is viewable from the outside.
meillo@117 28
meillo@117 29
meillo@120 30 \subsubsection*{Relay-only MTAs}
meillo@89 31 \label{subsec:relay-only}
meillo@217 32 Also called \name{forwarders}. 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. All they do is transfer mail to a specified smart host for further relay.
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meillo@89 34 Most \MTA{}s can be configured to act as such a \name{forwarder}. But this is usually an additional functionality.
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meillo@217 36 One uses this kind of \MTA\ 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 relay-only \MTA{}s, while there is one mail server that acts as a \name{smart host}. The ``dumb'' clients send mail to this \name{mail server} which does all further work.
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meillo@217 38 Example programs in that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.
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meillo@89 40
meillo@117 41 \subsubsection*{Groupware}
meillo@217 42 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 the actual functionality. Modules for mail transfer, file storage, calendars, resource management, instant messaging, and more, are commonly available.
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meillo@217 44 These program suites are used if the main work to do is providing integrated communication facilities and team working support for a group of people. Mail transfer is only one part of the problem to solve. 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.
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meillo@217 46 Examples for groupware are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org}, and \name{eGroupWare}.
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meillo@89 48
meillo@120 49 \subsubsection*{``Real'' MTAs}
meillo@217 50 There is a third type of \mta{}s in between the minimalistic \name{relay-only} \MTA{}s and the feature loaded \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.
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meillo@224 52 Common to them is their focus on transferring email, while being able to act as \name{smart host}s. Their variety ranges from ones mostly restricted to mail transfer (e.g.\ \qmail) to others having interfaces for adding further mail processing modules (e.g.\ \postfix). This group covers everything in between the other two groups.
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meillo@217 54 ``Real \MTA{}s'' are of importance in this document. All programs selected for the comparison in the following section are ``real \MTA{}s''. \masqmail\ is one too.
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meillo@89 56
meillo@117 57 \subsubsection*{Other segmenting}
meillo@124 58 \name{Mail transfer agents} can also be split in other ways.
meillo@217 59 Due to \sendmail's significance---described in section \ref{sec:sendmail}---compatibility interfaces for \sendmail\ are important for \unix\ \MTA{}s. Being not \emph{sendmail-compatible} may not 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?
meillo@124 60 \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
meillo@89 61
meillo@217 62 Another separation can be done between \freesw\ \MTA{}s and proprietary ones. 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?
meillo@217 63 Comparison with 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
meillo@217 64 need to be compared against other projects of similar shape. The document is seen from \masqmail's point of view---an \MTA\ for \unix\ systems on home servers and workstations---so non-free software is out of the way.
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meillo@120 69 \section{Popular MTAs}
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meillo@130 71 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.
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meillo@217 73 The programs chosen to be compared, with each other and with \masqmail, are: \sendmail, \exim, \qmail, and \postfix. They are the most important representatives of the regarded group.
meillo@117 74
meillo@145 75
meillo@145 76 \subsection{Market share analysis}
meillo@145 77
meillo@217 78 \MTA\ statistics are rare, differ, and good data is hard to collect. These points are bad if one wants good statistics. Thus it is obvious there are only few available.
meillo@217 79
meillo@248 80 Table \ref{tab:mta-market-share} shows the most used \MTA{}s determined by three different statistics. The first was done by \person{Daniel~J.\ Bernstein} (the author of \qmail) in 2001 \cite{bernstein01}. The second is by \person{Simpson} and \person{Bekman} in 2007 and was published on \name{O'ReillyNet} \cite{simpson07}. And the third is from \name{MailRadar.com} with unknown date\footnote{The footer of the website shows ``Copyright 2007'' but more likely does this refer to the whole website.} \citeweb{mailradar:mta-stats}.
meillo@117 81
meillo@130 82 \begin{table}
meillo@130 83 \begin{center}
meillo@130 84 \input{input/mta-market-share.tex}
meillo@130 85 \end{center}
meillo@130 86 \caption{Market share of \MTA{}s}
meillo@130 87 \label{tab:mta-market-share}
meillo@130 88 \end{table}
meillo@89 89
meillo@217 90 All surveys show high market shares for the four \MTA{}s: \sendmail, \exim, \qmail, and \postfix. Only the \name{Microsoft} mail server software and \name{IMail} have comparable large shares. Other \freesw\ \mta{}s (\name{smail}, \name{zmailer}, \name{MMDF}, \name{courier-mta}) are less important and seldom used.
meillo@130 91
meillo@217 92 The three surveys base on different data. \person{Bernstein} took 1\,000\,000 randomly chosen \NAME{IP} addresses, containing 39\,206 valid hosts; 958 of them accepted \NAME{SMTP} connections. The \person{Simpson} and \person{Bekman} survey used only domains owned by companies; in total 400\,000 hosts. \name{MailRadar} scanned 2\,818\,895 servers, leading to 59\,209 accepted connections.
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meillo@225 94 All surveys show \sendmail\ to be the most popular \MTA. \postfix, \qmail, and \exim\ are among the best seven in each. \exim\ has slightly smaller shares than the other two. The four together share more than half of the market according to \person{Bernstein} and the \name{MailRadar} statistics. \person{Simpson} and \person{Bekman} have their share to be somewhere between a third and the half. This uncertainty comes from the large amount of unidentifiable \MTA{}s.
meillo@143 95
meillo@225 96 The 22 percent of \name{mail security layers} in the \name{O'Reilly} survey is remarkable. Mail security layers are software guards between the network and the \mta\ that filter unwanted mail before it reaches the \MTA. This increases security by filtering malicious content and by blocking attacks against the \MTA. This large share may be a result of only regarding business mail servers. The problem concerning the survey is the disguise of the \mta\ working behind the security layer. It seems wrong to assume equal shares for the \MTA{}s behind the guards as for the unguarded \MTA{}s, because mail security layers will be more often used to guard weak \MTA{}s, as strong ones do not need them so much. This needs to be kept in mind when using the \name{O'Reilly} survey.
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meillo@225 98 The date of the \name{Mailradar} statistics is not mentioned with it; a mail to \name{Mailradar} asking for information was not replied, unfortunately. However, it seems quite sure that the statistics were published after 2001, caused by the \sendmail\ and \postfix\ shares. But to decide whether before or after the one from \name{O'Reilly} would be just guessing.
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meillo@145 100
meillo@145 101 \subsection{The four major Free Software MTAs}
meillo@143 102
meillo@248 103 Now follows a small introduction to the four programs chosen for comparison. \masqmail\ is not presented here, as it was already introduced in chapter \ref{chap:introduction}. Longer introductions, including analysis and comparison, were written by \person{Jonathan de Boyne Pollard} \cite{jdebp}.
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meillo@117 106
meillo@120 107 \subsubsection*{sendmail}
meillo@89 108 \label{sec:sendmail}
meillo@217 109 \sendmail\ is the best known \mta, since it was one of the first and surely the one that made \MTA{}s popular. It also was shipped as default \MTA{}s by many vendors of \unix\ systems. %fixme: ref
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meillo@248 111 The program was written by \person{Eric Allman} as the successor of his program \name{delivermail}. \person{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
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meillo@224 113 \sendmail\ designed to transfer mails between different protocols and networks, this lead to a very flexible, though complex, configuration.
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meillo@217 115 It was first released with \NAME{BSD} 4.1c in 1983. 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.
meillo@89 116
meillo@128 117 Further development will go into the project \name{MeTA1} (the former name was \name{sendmail X}) which succeeds \sendmail.
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meillo@217 119 More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and in the, so called, ``Bat Book'' \cite{costales97}.
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meillo@117 122
meillo@120 123 \subsubsection*{exim}
meillo@117 124 \label{sec:exim}
meillo@248 125 \exim\ was started in 1995 by \person{Philip Hazel} at the \name{University of Cambridge}. It is a fork of \name{smail-3}, and inherited a monolithic architecture similar to \sendmail's. But having no separation of the individual components of the system did not hurt. Its security is quite good. %fixme: ref
meillo@117 126
meillo@217 127 \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 checkers are provided by design. %fixme: ref
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meillo@117 129 The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007.
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meillo@217 131 One finds \exim\ on its homepage \citeweb{exim:homepage}. The standard literature is \person{Hazel}'s \exim\ book \cite{hazel01}.
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meillo@117 134
meillo@120 135 \subsubsection*{qmail}
meillo@89 136 \label{sec:qmail}
meillo@248 137 \qmail\ is seen by its community as ``a modern SMTP server which makes sendmail obsolete'' \citeweb{qmail:homepage2}. 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
meillo@89 138
meillo@223 139 \qmail\ first introduced many innovative concepts in \mta\ design. The most obvious contrast to \sendmail\ and \exim\ is its modular design. But \qmail\ was not the first modular \MTA. \NAME{MMDF}, which predates even \sendmail, was modular too. Regardless of \NAME{MMDF}'s modular architecture, \qmail\ is generally seen as the first security-aware \MTA. %fixme:ref
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meillo@225 141 The latest release of \qmail\ is version 1.03 from July 1998. In November 2007, afterwards, \qmail's source was put into the \name{public domain}. This makes it Free Software.
meillo@89 142
meillo@223 143 Because of \person{Bernstein}'s inactivity though changing requirements since 1998, ``[a] motley krewe of qmail contributors (see the README) has put together a netqmail-1.06 distribution of qmail. It is derived from Daniel Bernstein's qmail-1.03 plus bug fixes, a few feature enhancements, and some documentation.'' \citeweb{netqmail:homepage}.
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meillo@248 145 \qmail's homepages are \citeweb{qmail:homepage1} and \citeweb{qmail:homepage2}. The best book about \qmail, from \person{Bernstein}'s view, is \person{Dave Sill}'s handbook \cite{sill02}. His free available guide ``Life with qmail'' is another valuable source \cite{lifewithqmail}.
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meillo@117 148
meillo@120 149 \subsubsection*{postfix}
meillo@89 150 \label{sec:postfix}
meillo@248 151 The \postfix\ project 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.
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meillo@132 153 Today \postfix\ is taken by many \unix\ systems and \gnulinux\ distributions as default \MTA.
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meillo@132 155 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.
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meillo@217 157 Additional information can be retrieved from the program's homepage \citeweb{postfix:homepage}. \person{Dent}'s \postfix\ book \cite{dent04} claims to be ``the definitive guide'', and it is.
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meillo@120 164 \section{Comparison of MTAs}
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meillo@248 166 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} \cite{plug:mtas}. Tabular overviews may be found at \citeweb{mailsoftware42}, \citeweb{wikipedia:comparison-of-mail-servers}, and \cite[section 1.9]{lifewithqmail}.
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meillo@217 168 Here provided is an overview on important properties of the four previously introduced \MTA{}s. The data comes from the above stated sources and is collected in table \ref{tab:mta-comparison}.
meillo@126 169
meillo@117 170 \begin{table}
meillo@217 171 % FIXME: improve table data!!!
meillo@126 172 \begin{center}
meillo@126 173 \input{input/mta-comparison.tex}
meillo@126 174 \end{center}
meillo@126 175 \caption{Comparison of MTAs}
meillo@126 176 \label{tab:mta-comparison}
meillo@117 177 \end{table}
meillo@89 178
meillo@89 179
meillo@201 180 \subsubsection*{Architecture}
meillo@89 181
meillo@132 182 Architecture is most important when comparing \MTA{}s. Many other properties of a program depend on its architecture. %fixme: add ref?
meillo@248 183 \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 \cite{jdebp} is a source too.
meillo@89 184
meillo@132 185 Two different architecture types show off: monolithic and modular \mta{}s.
meillo@130 186
meillo@217 187 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 to be a security risk often. Thus it it should be avoided if possible.} binary which does all the work.
meillo@130 188
meillo@217 189 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} can be avoided.
meillo@130 190
meillo@248 191 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, it was the architecture that enabled \postfix\ to grow without running into security problems. \cite[page 13]{venema:postfix-growth}
meillo@130 192
meillo@217 193 The modular design, with each sub-program doing one part of the overall job, conforms to the \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.
meillo@130 194
meillo@132 195 Today modular \mta\ architectures are the state-of-the-art.
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meillo@89 197
meillo@217 198 \subsubsection*{Spam checking and content processing}
meillo@89 199
meillo@217 200 << FIXME >> % fixme
meillo@89 201
meillo@89 202
meillo@217 203 \subsubsection*{Future requirements}
meillo@89 204
meillo@217 205 In chapter \ref{chap:market-analysis}, it was tried to figure out trends and future requirements for \MTA{}s. The four programs are compared on these (possible) future requirements now.
meillo@126 206
meillo@225 207 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 cryptic and very complex (it has legendary 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 appears to be necessary 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 almost always contrary goals.
meillo@217 208
meillo@225 209 As second trend, the decreasing necessity 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, seems to be a good example.
meillo@217 210
meillo@217 211 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
meillo@217 212 But the modular architecture, used by \qmail\ and \postfix, is generally seen to be conceptually more secure, however. %fixme: ref
meillo@132 213 \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?
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meillo@93 216
meillo@193 217 \section{Result}
meillo@193 218
meillo@193 219 %fixme: write a result here
meillo@89 220
meillo@89 221
meillo@117 222
meillo@117 223
meillo@132 224 %todo: my own poll (?)
meillo@117 225
meillo@117 226
meillo@132 227 %<< 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 >>
meillo@117 228
meillo@117 229
meillo@132 230 %<< quality criteria >> << standards of any kind >> << how to compare? >> << (bewertungsmatrix) objectivity >> << how many criteria for ``good''? >>
meillo@133 231