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1 \chapter{Mail transfer agents}
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2
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3 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.
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4
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5
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6
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7
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8 \section{Types of MTAs}
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9 ``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.
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10
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11 This is how Bryan Costales defines a \mta\ in \cite{costales97}:
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12 \begin{quote}
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13 A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office.
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14 \end{quote}
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15 \name{The Free Dictionary} is a bit more concrete on the term: \citeweb{website:thefreedictionary}
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16 \begin{quote}
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17 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.
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18 \end{quote}
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19
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20 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.
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21
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22 Following is a classification of \mta{}s into groups of similar programs, regarding what is viewable from the outside.
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23
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24
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25 \subsubsection*{Relay-only MTAs}
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26 \label{subsec:relay-only}
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27 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.
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28
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29 Most \MTA{}s can be configured to act as such a \name{forwarder}. But this is usually an additional functionality.
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30
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31 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.
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32
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33 Examples for that group are: \name{nullmailer}, \name{ssmtp} and \name{esmtp}.
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34
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35
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36 \subsubsection*{Groupware}
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37 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.
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38
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39 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.
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40
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41 Examples are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.
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42
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43
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44 \subsubsection*{``Real'' MTAs}
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45 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.
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46
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47 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 (\name{qmail}) to others already having interfaces for adding further mail processing modules (\name{postfix}). They cover everything in between the other two groups. %FIXME: are postfix and qmail good examples?
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48
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49 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.
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50
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51
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52 \subsubsection*{Other segmenting}
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53 \name{Mail transfer agents} can also be split in other ways.
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54 \begin{itemize}
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55 \item
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56 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?
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57 \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
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58
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59 \item
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60 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?
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61 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
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62 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.
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63 \end{itemize}
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64
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65
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66
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67
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68 \section{Popular MTAs}
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69
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70 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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71
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72 The programs chosen are: \sendmail, \name{exim}, \name{qmail}, and \name{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.
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73
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74 Table \ref{tab:mta-market-share} shows the Top 10 \MTA{}s of three different statistics. The first published by \name{O'ReillyNet} in YYYY \citeweb{oreillynet:mta-stats} , the second by \name{Mailradar.com} from YYYY \citeweb{mailradar:mta-stats} , and the third by \textsc{Daniel~J.\ Bernstein} (the author of \name{qmail}) done in 2001 \citeweb{djb:mta-stats}.
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75
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76 \begin{table}
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77 \begin{center}
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78 \input{input/mta-market-share.tex}
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79 \end{center}
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80 \caption{Market share of \MTA{}s}
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81 \label{tab:mta-market-share}
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82 \end{table}
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83
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84 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.
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85
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86
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87 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 \textsc{Jonathan de Boyne Pollard} \citeweb{jdebp}.
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88
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89
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90
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91 \subsubsection*{sendmail}
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92 \label{sec:sendmail}
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93 \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
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94
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95 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. %fixme: ref
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96
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97 \sendmail\ is focused on transferring mails between different protocols and networks, this lead to a very flexible (though complex) configuration.
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98
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99 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.}.
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100
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101 Further development will go into the project \name{MeTA1} (the former name was \name{sendmail X}) which succeeds \sendmail.
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102
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103 More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage}.
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104
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105
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106
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107 \subsubsection*{exim}
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108 \label{sec:exim}
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109 \name{exim} was started in 1995 by 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 \name{qmail} and \name{postfix} have, did not hurt. Its security is comparably good. %fixme: ref
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110
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111 \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. %fixme: ref
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112
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113 The program is \freesw, released under the \GPL. The latest stable version is 4.69 from December 2007.
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114
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115 One finds \name{exim} on its homepage \citeweb{exim:homepage}.
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116
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117
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118
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119 \subsubsection*{qmail}
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120 \label{sec:qmail}
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121 \name{qmail} is seen by its community as ``a modern SMTP server which makes sendmail obsolete''.%fixme: ref
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122 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. %fixme: ref
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123
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124 \name{qmail} first introduced many innovative concepts in \mta\ design and is generally seen as the first security-aware \MTA\ developed. %fixme:ref
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125 %fixme: what about mmdf?
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126
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127 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.
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128
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129 The programs homepages are \citeweb{qmail:homepage1} and \citeweb{qmail:homepage2}. Further information about \name{qmail} is available with Dave Sill's ``Life with qmail'' \citeweb{lifewithqmail}.
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130
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131
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132
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133 \subsubsection*{postfix}
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134 \label{sec:postfix}
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135 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.
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136
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137 Today \name{postfix} is taken by many \unix\ systems and \gnulinux\ distributions as default \MTA.
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138
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139 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.
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140
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141 Additional information is available on the program's homepage \citeweb{postfix:homepage}.
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142
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143
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144
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145
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146
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147
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148 \section{Comparison of MTAs}
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149
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150 This section tries not to provide an overall \MTA\ comparison, because this is already done by others. Remarkable are the one by Shearer \cite{shearer06} and an email discussion on the mailing list \name{plug@lists.q-linux.com} \citeweb{plug:mtas}. Tabulary overviews may be found at \citeweb{mailsoftware42} and \citeweb{wikipedia:comparison-of-mail-servers}.
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151
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152 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}.
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153
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154
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155
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156 \begin{table}
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157 \begin{center}
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158 \input{input/mta-comparison.tex}
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159 \end{center}
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160 \caption{Comparison of MTAs}
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161 \label{tab:mta-comparison}
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162 \end{table}
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163
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164
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165
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166 \subsection{About architecture}
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167
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168 Hafiz \cite{hafiz05} discusses in detail on \mta\ architecture (comparing \sendmail, \name{qmail}, \name{postfix}, and \name{sendmail X}).
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169
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170
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171 \url{http://archives.neohapsis.com/archives/postfix/2006-07/1762.html} %sloc evolution of postfix, sendmail, qmail
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172
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173
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174
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175 \subsection{Security comparison}
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176
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177
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178
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179
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180
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181
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182 \paragraph{Ref back to \ref{sec:what-will-be-important}}
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183
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184 provider indepencence -> easy config:
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185 \sendmail\ and \name{qmail} appear to have bad positions at this point. Their configuration is complex, thus they would need simplification wrappers around them to provide easy configuration.
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186
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187 performance not so important:
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188 \name{postfix} focuses much on performance, this might not be an important point then.
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189
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190 security:
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191 It seems as if all widely used \mta{}s provide good security nowadays. \name{qmail}'s architecture, also used in \name{postfix}, is generally seen to be conceptually more secure, however.
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192
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193
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194
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195 \paragraph{local mail delivery}
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196 But for example delivery of mail to local users is \emph{not} what \mta{}s should care about, although most \MTA\ are able to deliver mail, and many do. (\name{mail delivery agents}, like \name{procmail} and \name{maildrop}, are the right programs for this job.)
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197
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198
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199 \paragraph{various protocols}
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200 protocols like \NAME{SMTP} and \NAME{UUCP}, between which mail is transferred.\footnote{\sendmail{}'s initial purpose was moving mail between \NAME{UUCP}, \NAME{SMTP}, and \name{Berknet}.}
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201
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208
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209 << 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 >>
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210
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211
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212 << quality criteria >> << standards of any kind >> << how to compare? >> << (bewertungsmatrix) objectivity >> << how many criteria for ``good''? >>
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