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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. 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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8 \section{Types of \MTA{}s} 8 \section{Types of MTAs}
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. 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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11 This is how Bryan Costales defines a \mta\ in \cite{costales97}: 11 This is how Bryan Costales defines a \mta\ in \cite{costales97}:
12 \begin{quote} 12 \begin{quote}
13 A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office. 13 A mail transfer agent (MTA) is a highly specialized program that delivers mail and transports it between machines, like the post office.
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. 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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22 Following is a classification of \mta{}s into groups of similar programs, regarding what is viewable from the outside. 22 Following is a classification of \mta{}s into groups of similar programs, regarding what is viewable from the outside.
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24 24
25 \subsubsection*{Relay-only \MTA{}s} 25 \subsubsection*{Relay-only MTAs}
26 \label{subsec:relay-only} 26 \label{subsec:relay-only}
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. 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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29 Most \MTA{}s can be configured to act as such a \name{forwarder}. But this is usually an additional functionality. 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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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 efficently. But one may use \name{groupware} on the home server for his family members also. 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 efficently. But one may use \name{groupware} on the home server for his family members also.
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41 Examples are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}. 41 Examples are: \name{Lotus Notes}, \name{Microsoft Exchange}, \name{OpenGroupware.org} and \name{eGroupWare}.
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44 \subsubsection*{``Real'' \MTA{}s} 44 \subsubsection*{``Real'' MTAs}
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. 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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47 Common to them is their focus on transfering 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? 47 Common to them is their focus on transfering 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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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. 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.
63 \end{itemize} 63 \end{itemize}
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67 67
68 \section{Popular \MTA{}s} 68 \section{Popular MTAs}
69 69
70 %todo: include market share analyses here 70 %todo: include market share analyses here
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72 One would not use a program for a job it is not suited for. Therefor only \mta{}s that are mostly similar to \masqmail\ are regarded here. These are \emph{sendmail-compatible} ``smart'' \freesw\ \MTA{}s that focus on mail transfer. 72 One would not use a program for a job it is not suited for. Therefor only \mta{}s that are mostly similar to \masqmail\ are regarded here. These are \emph{sendmail-compatible} ``smart'' \freesw\ \MTA{}s that focus on mail transfer.
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80 Following is a small introduction to each of the five programs chosen for comparision, except \masqmail\ which already was intoduced in chapter \ref{chap:introduction}. 80 Following is a small introduction to each of the five programs chosen for comparision, except \masqmail\ which already was intoduced in chapter \ref{chap:introduction}.
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83 83
84 \subsubsection*{\sendmail} 84 \subsubsection*{sendmail}
85 \label{sec:sendmail} 85 \label{sec:sendmail}
86 \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 86 \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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88 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 88 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 More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and on \citeweb{wikipedia:sendmail} and \citeweb{jdebp}. 96 More information can be found on the \sendmail\ homepage \citeweb{sendmail:homepage} and on \citeweb{wikipedia:sendmail} and \citeweb{jdebp}.
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100 \subsubsection*{\name{exim}} 100 \subsubsection*{exim}
101 \label{sec:exim} 101 \label{sec:exim}
102 \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 monolitic 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 102 \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 monolitic 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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104 \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 104 \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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108 One finds \name{exim} on its homepage \citeweb{exim:homepage}. More information about it can be retrieved from \citeweb{wikipedia:exim} and \citeweb{jdebp}. 108 One finds \name{exim} on its homepage \citeweb{exim:homepage}. More information about it can be retrieved from \citeweb{wikipedia:exim} and \citeweb{jdebp}.
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111 111
112 \subsubsection*{\name{qmail}} 112 \subsubsection*{qmail}
113 \label{sec:qmail} 113 \label{sec:qmail}
114 \name{qmail} is seen by its community as ``a modern SMTP server which makes sendmail obsolete''.%fixme: ref 114 \name{qmail} is seen by its community as ``a modern SMTP server which makes sendmail obsolete''.%fixme: ref
115 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 115 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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117 \name{qmail} first introduced many innovative concepts in \mta\ design and is generally seen as the first security-aware \MTA\ developed. %fixme:ref 117 \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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122 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}. 122 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}.
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126 \subsubsection*{\name{postfix}} 126 \subsubsection*{postfix}
127 \label{sec:postfix} 127 \label{sec:postfix}
128 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. 128 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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130 Today \name{postfix} is taken by many \unix\ systems and \gnulinux\ distributions as default \MTA. 130 Today \name{postfix} is taken by many \unix\ systems and \gnulinux\ distributions as default \MTA.
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141 \section{Comparison of \MTA{}s} 141 \section{Comparison of MTAs}
142 142
143 << general fact in table \ref{tab:mta-comparison} >> 143 << general fact in table \ref{tab:mta-comparison} >>
144 144
145 \begin{table} 145 \begin{table}
146 \begin{tabular}[hbt]{| p{0.13\textwidth} || p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} |} 146 \begin{tabular}[hbt]{| p{0.13\textwidth} || p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} | p{0.13\textwidth} |}