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   11   <title>The Courier Mail Server</title>
   12   <link rel="icon" href="icon.gif" type="image/gif" />
   13 </head>
   15 <body>
   16   <p class="toc"><a href="#features">Features</a><br />
   17   <a href="install.html">Install</a></p>
   19   <p>The <em>Courier</em> mail transfer agent (MTA) is an
   20   integrated mail/groupware server based on open commodity
   21   protocols, such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP.
   22   <em>Courier</em> provides ESMTP, IMAP, POP3, webmail, and mailing
   23   list services within a single, consistent, framework. Individual
   24   components can be enabled or disabled at will. The
   25   <em>Courier</em> mail server now implements basic web-based
   26   calendaring and scheduling services integrated in the webmail
   27   module. Advanced groupware calendaring services will follow
   28   soon.</p>
   30   <p>The <em>Courier</em> mail server's source code should compile
   31   on most POSIX-based operating systems based on Linux, and
   32   BSD-derived kernels. The <em>Courier</em> mail server should also
   33   compile on Solaris and AIX, with some help from Sun's or IBM's
   34   freeware add-on tools for their respective operating systems.</p>
   36   <p>The <em>Courier</em> mail server evolved out of several
   37   related projects, that merged together (more on that later). The
   38   <em>Courier</em> mail server implements SMTP extensions for
   39   mailing list management and spam filtering. The <em>Courier</em>
   40   mail server can function as an intermediate mail relay, relaying
   41   mail between an internal LAN and the Internet, or perform final
   42   delivery to mailboxes. The <em>Courier</em> mail server uses
   43   <a target="_blank" href=
   44   "http://www.courier-mta.org/mbox-vs-maildir/" shape=
   45   "rect">maildirs</a> as its native mail storage format, but it can
   46   also deliver mail to legacy mailbox files as well. The
   47   <em>Courier</em> mail server's configuration is set by plain text
   48   files and Perl scripts. Most of The <em>Courier</em> mail
   49   server's configuration can now be adjusted from a web browser,
   50   using The <em>Courier</em> mail server's web-based administration
   51   module.</p>
   53   <p>The <em>Courier</em> mail server can provide mail services for
   54   regular operating system accounts. The <em>Courier</em> mail
   55   server can also provide mail services for virtual mail accounts,
   56   managed by an LDAP, MySQL, or PostgreSQL-based authentication
   57   database.</p>
   59   <p>Certain portions of the <em>Courier</em> mail server -
   60   <a target="_top" href="http://www.courier-mta.org/maildrop/"
   61   shape="rect">the mail filtering engine</a>, the <a href=
   62   "http://www.courier-mta.org/sqwebmail/" target="_top" shape=
   63   "rect">webmail server</a> and the <a target="_top" href=
   64   "http://www.courier-mta.org/imap/" shape="rect">IMAP server</a> -
   65   are also available as separate, smaller, packages that can be
   66   used with other mail servers.</p>
   68   <h2><a name="features" id="features"></a>Features</h2>
   70   <ul>
   71     <li>Can be configured to function as an intermediate mail
   72     relay, or as a mail server that receives mail for multiple
   73     domains and makes it accessible to mail clients, or anything in
   74     between.</li>
   76     <li>On servers with multiple IP addresses, optionally assign a
   77     <a href="courier.html#multihomed">vanity configuration to
   78     <em>Courier</em> for each IP address</a>, making each IP
   79     address look like a separate, dedicated, mail server instance,
   80     for both incoming and outgoing mail. An alternative limited
   81     vanity configuration for outgoing mail only, based on the
   82     sending mail client's authenticated login, is possible when
   83     multiple IP addresses are not available.</li>
   85     <li>Web-based administration and configuration tool.</li>
   87     <li>Local mailboxes can be accessed via POP3. The
   88     <em>Courier</em> mail server includes an integrated POP3
   89     server.</li>
   91     <li>Local mailboxes can be accessed via IMAP. The
   92     <em>Courier</em> mail server includes an integrated IMAP
   93     server.</li>
   95     <li>A built-in IMAP/POP3 aggregator proxy. It is possible to
   96     distribute all mailboxes between multiple servers. A separate
   97     server (or a pool of servers) accepts connections from IMAP or
   98     POP3 clients, then connects to the right server based on the
   99     mailbox the connecting client is logging into.</li>
  101     <li>Local mailboxes can be accessed via HTTP. The
  102     <em>Courier</em> mail server includes an <a href=
  103     "/sqwebmail/screenshots.html">integrated webmail
  104     server</a>.</li>
  106     <li>The webmail server includes a personal <a target="_blank"
  107     href=
  108     "http://www.courier-mta.org/sqwebmail/images/monthly.png">event
  109     calendar</a>.</li>
  111     <li>Uses an efficient <a target="_blank" href=
  112     "http://www.courier-mta.org/mbox-vs-maildir/">maildir</a>
  113     format as its native mail storage format. Some support is
  114     provided for legacy mbox mailboxes.</li>
  116     <li>Flexible "Sender Policy Framework" support; the ESMTP
  117     <code>HELO</code>, <code>MAIL FROM</code>, and the
  118     <code>From:</code> header can be validated using SPF.</li>
  120     <li>DSN, PIPELINING, and 8BITMIME ESMTP extensions. The
  121     <em>Courier</em> mail server automatically converts 8-bit
  122     messages to 7-bit encoding, for relaying mail to external mail
  123     gateways.</li>
  125     <li>STARTTLS ESMTP extension (as well as
  126     IMAP/POP3/ESMTP/Webmail over SSL) in both the client and the
  127     server (requires OpenSSL). The ESMTP client can optionally
  128     require that the remote server's X.509 certificate is signed by
  129     a trusted root CA (a default set of root CAs is provided).</li>
  131     <li>Experimental TLS/SSL enhancements which are designed to
  132     implement a secure mail delivery channel between trusted
  133     domains, over an untrusted network. This is implemented by
  134     requiring mail to select domains use TLS/SSL connections which
  135     require the remote server to present an X.509 certificate
  136     signed by a private (not a public) certificate authority. This
  137     is pretty much the highest level of security that can be
  138     achieved with today's technologies. This doesn't even require
  139     DNSsec. Even if the DNS cache is poisoned with MX records that
  140     divert mail to a rogue relay, the attacker will not have an
  141     X.509 certificate signed by a private CA (this assumes, of
  142     course, that the security of the private CA hasn't been
  143     breached). This work is mostly complete, but still needs a
  144     little testing.</li>
  146     <li>Message submission protocol (RFC 2476).</li>
  148     <li>IPv6 support (experimental).</li>
  150     <li>NOTE: the integrated servers work with maildir-based
  151     mailboxes only. There are many existing POP3, IMAP, and webmail
  152     servers that provide excellent support for mbox-based
  153     mailboxes, so there's no reason to reinvent the wheel. Some
  154     popular mbox servers are: <a target="_blank" href=
  155     "http://www.eudora.com/qpopper/">Qpopper</a>, <a target=
  156     "_blank" href="http://www.washington.edu/imap/">UW-IMAP</a>,
  157     and <a target="_blank" href=
  158     "http://neomail.sourceforge.net/">NeoMail</a>.</li>
  160     <li>A faxmail gateway (experimental) that forwards E-mail
  161     messages via fax (requires a compatible class 2 faxmodem). The
  162     <em>Courier</em> mail server doesn't implement the actual
  163     faxing all by itself, actually. The <em>Courier</em> mail
  164     server uses additional software (which must be separately
  165     installed), to take care of the low-level details. The popular
  166     <a target="_blank" href=
  167     "http://alpha.greenie.net/mgetty/">mgetty+sendfax</a> package
  168     talks to the faxmodem and handles the actual faxing. Conversion
  169     of E-mail messages to fax pages is done by <a target="_blank"
  170     href="http://www.ghostscript.com/">ghostscript</a>, troff or
  171     <a target="_blank" href=
  172     "http://www.gnu.org/software/groff/groff.html">groff</a>, and
  173     the <a target="_blank" href=
  174     "http://netpbm.sourceforge.net/">NetPBM</a> library. The
  175     <em>Courier</em> mail server glues all of these pieces together
  176     in a seamless manner any time an E-mail message addressed to
  177     <code>&lt;phonenumber@fax&gt;</code> is received. The main
  178     textual body of the E-mail message is placed on a cover page,
  179     and any attachments are converted to fax image format and
  180     transmitted after the cover page. At this time, The
  181     <em>Courier</em> mail server knows how to send plain text, PDF,
  182     and Postscript attachments. GIF, JPEG, and PNG images can be
  183     sent to (one image per page). The additional software packages
  184     that were mentioned previously are usually already included in
  185     most Linux and BSD installations. In most cases no additional
  186     software really needs to be installed in order to get
  187     faxmailing up and running.</li>
  189     <li>The <em>Courier</em> mail server includes a mailing list
  190     manager, with fully automatic bounce processing.</li>
  192     <li>You don't need a full-blown mail server? <em>Courier</em>
  193     mail server's <a target="_top" href=
  194     "http://www.courier-mta.org/imap/">IMAP server</a>, <a target=
  195     "_top" href="http://www.courier-mta.org/sqwebmail/">webmail
  196     server</a>, and <a target="_top" href=
  197     "http://www.courier-mta.org/maildrop/">mail filter</a> are
  198     available as independent packages that can be used with other
  199     mail servers (as long as the other mail servers store mail in
  200     <a target="_blank" href=
  201     "maildir.html">maildirs</a>).
  202     These sub-packages are assembled from the same source code
  203     tree. The only difference is the top level makefile. Note: the
  204     independent builds are not always in sync with the main the
  205     <em>Courier</em> mail server build at any given time. They
  206     follow their own schedule, and may include a slightly older, or
  207     even more recent, code base! Over time, however, everything
  208     always syncs together since all builds are assembled from the
  209     same source code repository.</li>
  211     <li>SOCKSv5 support. The <em>Courier</em> mail server can punch
  212     through a SOCKS firewall to send outgoing mail. Receiving mail
  213     through a SOCKS firewall is not yet supported. To use SOCKS you
  214     need to install the <em>Courier</em> mail server's Socks 5
  215     proxy client library.</li>
  217     <li>PAM, LDAP, PostgreSQL (beta), or MySQL authentication. LDAP
  218     authentication requires <a target="_blank" href=
  219     "http://www.openldap.org">OpenLDAP</a> to be installed.
  220     LDAP-based mail routing is also supported.</li>
  222     <li>Gateway mail to/from UUCP (if compatible UUCP software is
  223     separately installed).</li>
  225     <li>Authenticated SMTP.</li>
  227     <li><code>XVERP</code> and <code>XEXDATA</code> ESMTP
  228     extensions.</li>
  230     <li>DNS-based blacklists. Ability to exempt whitelisted IP
  231     addresses from the blacklists.</li>
  233     <li>Integrated mail filtering. An API is provided for
  234     installing arbitrary external mail filters, and the system
  235     administrator can selectively enable for any mail source
  236     (ESMTP, UUCP, locally submitted mail) for filtering. Two
  237     example mail filters are included - one written in C that uses
  238     threads, and a Perl-based filter. The system administrator can
  239     also enable the ability for individual mail recipients to
  240     specify their own mail filtering rules, using a scripting
  241     language (implemented by <code>maildrop</code>, see below).
  242     Mail filtering is implemented as an integral part of the mail
  243     server. Unwanted mail is rejected, and is not accepted by the
  244     <em>Courier</em> mail server for delivery (the external mail
  245     relay receives the error, and it becomes the external relay's
  246     problem as to what to do with unwanted junk mail).</li>
  248     <li>Partial ability to import <code>sendmail</code>'s aliases
  249     file, but not all aspects of <code>sendmail</code>'s aliasing
  250     is supported - like delivering to programs, for example. Still,
  251     most simple <code>aliases</code> files should be usable.</li>
  253     <li>Optional ability to import most of <code>Qmail</code>'s
  254     <code>.qmail</code> files (<em>Courier</em> mail server uses an
  255     almost 100% compatible local mail delivery instruction
  256     format).</li>
  258     <li>Most major components of the <em>Courier</em> mail server
  259     can be installed in non-default directories, allowing extreme
  260     customization for your particular environment.</li>
  262     <li>You can set a maximum number of messages to deliver
  263     simultaneously to the same host. This, in fact, is strongly
  264     encouraged so that a single nonfunctioning domain does not take
  265     up all available delivery slots. Rate limiting is implemented
  266     in the main scheduler, and applies to any transport mechanism,
  267     not just ESMTP.</li>
  269     <li>Mailing list administrators can specify a backup relay and
  270     have mail that's not immediately deliverable offloaded to a
  271     backup server (this feature needs testing/feedback).</li>
  272   </ul>
  274   <p>However, it is also important to note what the
  275   <em>Courier</em> mail server does not have or will not
  276   support:</p>
  278   <ul>
  279     <li><code>.forward</code> files are partially supported. The
  280     <em>Courier</em> mail server can import most basic
  281     <code>/etc/aliases</code> files from <a target="_blank" href=
  282     "http://www.sendmail.org">sendmail</a>, but sendmail's
  283     <code>.forward</code> and <code>/etc/aliases</code> files are
  284     simply not 100% compatible with the <em>Courier</em> mail
  285     server's security model. Most <code>.forward</code> and
  286     <code>/etc/aliases</code> files should be acceptable, but some
  287     may not.</li>
  289     <li><code>ETRN</code> is not, and will never be implemented.
  290     It's a hack, and is functionally incompatible with the
  291     <em>Courier</em> mail server's internal message dispatcher. If
  292     a mail node does not have constant network connectivity, there
  293     are better ways of arranging for mail transport than ETRN. The
  294     transient mail node should download mail via IMAP, or maybe
  295     even UUCP.<br /></li>
  297     <li>Workarounds for known defects in other mail software. The
  298     <em>Courier</em> mail server will not accept mail with raw
  299     8-bit characters in the headers, because they are illegal.
  300     There are well-defined protocols that must be used to encode
  301     8-bit text in mail headers. Non-compliant messages may result
  302     in the <em>Courier</em> mail server itself issuing corrupted
  303     delivery status notifications, or mishandling the message in
  304     several other ways. Because of that corrupted mail will simply
  305     not be accepted. Neither will the <em>Courier</em> mail server
  306     deliver mail to domains with improperly-defined MX records,
  307     even though other mail servers ignore the bad data.
  308     Additionally, certain popular IMAP mail clients are known to
  309     not work with the <em>Courier</em> mail server's IMAP server,
  310     due to an improper IMAP implementation by the mail client.</li>
  312     <li>Scripting language for rewriting mail headers. Mail
  313     rewriting rules are hardcoded, and are expected to be
  314     sufficient in most cases. If you have an unusual situation that
  315     requires some oddball header rewriting, you'll have to
  316     implement it yourself.</li>
  318     <li>Support for mbox mailboxes in the POP3, IMAP, and webmail
  319     components. They support maildirs only. There are plenty of
  320     existing servers out there that read mbox mailboxes.</li>
  321   </ul>
  323   <h2>Requirements</h2>
  325   <ul>
  326     <li>A C++ compiler, <code>egcs</code> is recommended. Most of
  327     the <em>Courier</em> mail server are written in C, but several
  328     major sections are written in C++.</li>
  330     <li>GNU make. Other <code>make</code>s may work, but that's not
  331     guaranteed.</li>
  333     <li>Either the GDBM or Berkeley DB library must be available.
  334     Only certain versions of Berkeley DB API are supported, because
  335     the Berkeley DB API often changes (tested with 2.4.14 and
  336     1.8.5). GDBM is the recommended library.</li>
  338     <li>Perl 5.</li>
  340     <li>The file system must support FIFOs. At least the file
  341     system that stores the mail queue must be able to support
  342     FIFOs. The <em>Courier</em> mail server will not work with
  343     AFS.</li>
  345     <li>Filesystem domain sockets must be available.</li>
  347     <li>Some optional components have additional dependencies -
  348     notably the additional software required for faxmail support
  349     (see above).</li>
  350   </ul>
  352   <h2>Additional information</h2>Here is a somewhat more detailed
  353   overview of the <em>Courier</em> mail server's less prominent
  354   features:
  356   <h3>Upgrade path</h3>The <em>Courier</em> mail server can be
  357   installed on systems that were previously running sendmail or
  358   Qmail. Please note that the <em>Courier</em> mail server will be
  359   able to support most major features of both servers, however the
  360   <em>Courier</em> mail server is not, and will never be a
  361   100%-compatible replacement for either sendmail or Qmail. The
  362   <em>Courier</em> mail server does not implement several legacy
  363   features of either MTA, and there are no plans to implement them
  364   in the future. The key differences are:
  366   <ul>
  367     <li>sendmail</li>
  368   </ul>
  370   <p>A local mail delivery agent, such as <code>procmail</code>,
  371   should be used for maximum compatibility with sendmail.</p>
  373   <p>The <em>Courier</em> mail server expects system mailboxes to
  374   be in the users' home directories. If your system mailboxes are
  375   all stored separately, in <code>/var/spool/mail</code> or
  376   somewhere else, you'll need to use a local delivery agent such as
  377   <code>procmail</code>.</p>
  379   <p>The <em>Courier</em> mail server uses a filesystem lock on
  380   mailbox files, The <em>Courier</em> mail server does not support
  381   old-fashioned dot-locking. If you need dot-locking, use
  382   <code>procmail</code> or <code>maildrop</code> (included).</p>
  384   <ul>
  385     <li>Qmail</li>
  386   </ul>
  388   <p>A configuration switch allows the <em>Courier</em> mail server
  389   to read <code>$HOME/.qmail</code> files, however the
  390   <em>Courier</em> mail server's implementation is not 100%
  391   identical to Qmail's. The <em>Courier</em> mail server's
  392   <code>aliases</code> file is also used to implement Qmail-style
  393   virtual domains. A simple Perl script can be used to convert
  394   Qmail's <code>control/virtualdomains</code> into
  395   <code>aliases</code> entries.</p>
  397   <p>The <em>Courier</em> mail server supports Maildirs
  398   natively.</p>
  400   <p>The <em>Courier</em> mail server can use the
  401   <code>maildrop</code> mail filter as a local mail delivery agent.
  402   <code>maildrop</code> is optional, but, if used, The
  403   <em>Courier</em> mail server will take advantage of certain
  404   <code>maildrop</code>-specific features which optimize local mail
  405   delivery.</p>
  407   <h2>Mail filters</h2>The <em>Courier</em> mail server has hooks
  408   for optional, site-defined, mail filters. You'll have to write
  409   them yourself, though. The administrator-defined mail filters can
  410   block the message from being accepted by the <em>Courier</em>
  411   mail server (if messages comes in via SMTP, the SMTP server will
  412   reject it). The <em>Courier</em> mail server can also be
  413   configured to pause for a short period of time before attempting
  414   to deliver a message. If the mail filter detects a slew of
  415   duplicate messages coming in, the mail filter can block all
  416   future copies, and manually bounce the handful of copies from the
  417   queue. The system administrator can selectively enable filtering
  418   for any individual mail source (ESMTP, locally submitted mail,
  419   UUCP). The system administrator can also optionally enable
  420   recipient-specified mail filters. With recipient-specified mail
  421   filtering enabled, any local mail recipient can install an
  422   arbitrary mail filter to selectively accept or reject mail based
  423   on any criteria.
  425   <p>Currently the mail filtering API is not very well documented,
  426   but it's there.</p>
  428   <h2>ESMTP extensions</h2>The <em>Courier</em> mail server
  429   implements <code>AUTH</code>, <code>PIPELINING</code>,
  430   <code><a href="courierdsn.html">DSN</a></code>,
  431   <code>SIZE</code>, and <code>8BITMIME</code> extensions to SMTP.
  432   The <em>Courier</em> mail server also includes a reference
  433   implementation of the experimental <code>XVERP</code> and
  434   <code>XEXDATA</code> extensions.
  436   <p>The <em>Courier</em> mail server is a closed mail relay by
  437   default. The <em>Courier</em> mail server cannot be accidentally
  438   configured as a completely open relay. A deliberate feat of
  439   stupidity is required for that to happen.</p>
  441   <h2>ESMTP BOFH</h2>The <em>Courier</em> mail server does not
  442   deliver mail to domains with broken MX records. The
  443   <em>Courier</em> mail server also refuses to accept any mail with
  444   a return address in a domain with broken MX records.
  446   <p>The <em>Courier</em> mail server can automatically blacklist
  447   domains whose mail servers reject delivery status
  448   notifications.</p>
  450   <h2>Header rewriting</h2>The <em>Courier</em> mail server will
  451   rewrite headers and MIME-ify messages whenever appropriate.
  452   Header rewriting logic is hardcoded in C, there is no header
  453   rewriting language as in sendmail. An interpreted language
  454   imposes a drastic speed penalty. The rewriting library is fairly
  455   simple, and the the standard rewriting rules will do for most
  456   situations.
  458   <p>The <em>Courier</em> mail server rejects messages with
  459   badly-formed or missing MIME headers. The <em>Courier</em> mail
  460   server rejects messages containing 8-bit characters in the
  461   headers, or messages that contain 8-bit content, but do not have
  462   the required MIME headers. Accepting malformed messages of that
  463   kind can result in the <em>Courier</em> mail server itself
  464   sending mail that violates the relevant RFCs, therefore
  465   <em>Courier</em> mail server will simply reject
  466   improperly-formatted messages. There are well-defined RFC
  467   standards that explicitly spell out how mail containing 8-bit
  468   content or 8-bit headers should be encoded, and those standards
  469   will have to be properly implemented by anyone that wishes their
  470   mail to be accepted.</p>
  472   <h2>Modularity</h2>Message scheduling, dispatching, and the
  473   actual transport mechanism are completely modularized. Different
  474   message transport mechanisms such as UUCP can be implemented in a
  475   simple plug-in fashion, however some C coding will be required.
  477   <h2>Message scheduling</h2>The <em>Courier</em> mail server
  478   supports VERPs, multiple recipients per message, and
  479   RFC1894-compliant delivery status notifications.
  481   <h2>Load limiting</h2>You can set a maximum number of messages to
  482   deliver simultaneously to the same host. This, in fact, is
  483   strongly encouraged so that a single nonfunctioning domain does
  484   not take up all available delivery slots. Rate limiting is
  485   implemented in the main scheduler, and applies to any transport
  486   mechanism, not just ESMTP.
  488   <h2>Automatic restarts and garbage cleanup</h2>The
  489   <em>Courier</em> mail server's scheduling engine restarts itself
  490   automatically, on a regular basis. This helps with memory
  491   fragmentation. The <em>Courier</em> mail server tries to restart
  492   itself during periods of system inactivity.
  494   <h2>Smart organization of the message queue and temporary
  495   directories</h2>The <em>Courier</em> mail server automatically
  496   creates subdirectories when necessary, and deletes them when
  497   they're empty. When there's a sudden peak in the number of
  498   messages added to the queue, directories used to store the
  499   messages grow in size to accomodate the additional entries. On
  500   many file systems, once those messages are deleted, the empty
  501   slack space in the directory is not reclaimed, and actually slows
  502   down subsequent directory operations. The <em>Courier</em> mail
  503   server automatically removes empty directories, reclaiming the
  504   slack space.
  506   <h2>Smart installation layout</h2>
  508   <p>The <em>Courier</em> mail server's configuration script will
  509   install the <em>Courier</em> mail server into
  510   <code>/usr/lib/courier</code> by default. Everything will go into
  511   several subdirectories there: the actual binaries, configuration
  512   files, the mail queue, manual pages, and auxiliary support files.
  513   Optional configuration switches allow pretty much every major
  514   subdirectory to be relocated anywhere else. For example, the Red
  515   Hat RPM package for the <em>Courier</em> mail server relocates
  516   the configuration files to <code>/etc/courier</code>, the manual
  517   pages to <code>/usr/man</code>, and the mail queue to
  518   <code>/var/spool/courier</code>.</p>
  520   <h2>Mailing lists</h2>
  522   <p>The <em>Courier</em> mail server can implement both sendmail
  523   and qmail-style address aliases. De-duping of sendmail-style
  524   aliases is automatic. The <em>Courier</em> mail server source
  525   distribution also includes a complete mailing list manager.</p>
  526   <hr />
  528   <p class="copyright">Copyright 1998-2020 Double Precision, Inc.
  529   This software is distributed under the terms of the GNU General
  530   Public License. See COPYING for additional information.</p>
  531 </body>
  532 </html>