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<title>Introduction</title>
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<h2>Introduction</h2>
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Courier is a modular multiprotocol mail server that's designed to strike a
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balance between reasonable performance, flexibility and features.
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balance between reasonable performance, flexibility and features.
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<li>Can be configured to function as an intermediate mail relay, or as a
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mail server that receives mail for multiple domains and makes it
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accessible to mail clients, or anything in between.<br />
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<li><a href="/webadmin/">Web-based administration and configuration
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<li>Uses an efficient <a
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accessible to mail clients, or anything in between.</li>
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<li><a target="_blank" href="/webadmin/">Web-based administration and
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configuration tool</a> .</li>
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<li>Local mailboxes can be accessed via POP3. Courier includes an
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integrated POP3 server.</li>
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<li>Local mailboxes can be accessed via IMAP. Courier includes an
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integrated IMAP server.</li>
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<li>Local mailboxes can be accessed via HTTP. Courier includes an <a
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href="/sqwebmail/screenshots.html">integrated webmail server</a>.</li>
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<li>The webmail server includes a personal <a target="_blank"
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href="http://www.courier-mta.org/sqwebmail/images/monthly.png">event
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<li>Uses an efficient <a target="_blank"
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href="http://www.courier-mta.org/mbox-vs-maildir/">maildir</a> format as
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its native mail storage format. Some support is provided for legacy mbox
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<li>Flexible "Sender Policy Framework" support; the ESMTP
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<code>HELO</code>, <code>MAIL FROM</code>, and the <code>From:</code>
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header can be validated using SPF.</li>
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<li>DSN, PIPELINING, and 8BITMIME ESMTP extensions. Courier automatically
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converts 8-bit messages to 7-bit encoding, for relaying mail to external
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<li>STARTTLS ESMTP extension (as well as IMAP/POP3/ESMTP/Webmail over SSL)
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in both the client and the server (requires OpenSSL). The ESMTP client
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can optionally require that the remote server's X.509 certificate is
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signed by a trusted root CA (a default set of root CAs is provided).<br />
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signed by a trusted root CA (a default set of root CAs is provided).</li>
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<li>Experimental TLS/SSL enhancements which are designed to implement a
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secure mail delivery channel between trusted domains, over an untrusted
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network. This is implemented by requiring mail to select domains use
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the attacker will not have an X.509 certificate signed by a private CA
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(this assumes, of course, that the security of the private CA hasn't been
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breached). This work is mostly complete, but still needs a little
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<li>Message submission protocol (RFC 2476).<br />
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<li>IPv6 support (experimental).<br />
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<li>Mailboxes can be accessed via POP3. Courier source distribution
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includes an integrated POP3 server.<br />
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<li>Mailboxes can be accessed via IMAP. Courier source distribution
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includes an integrated IMAP server.<br />
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<li>Mailboxes can be accessed via HTTP. Courier source distribution
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includes an integrated webmail server.<br />
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<li>Message submission protocol (RFC 2476).</li>
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<li>IPv6 support (experimental).</li>
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<li>NOTE: the integrated servers work with maildir-based mailboxes only.
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There are many existing POP3, IMAP, and webmail servers that provide
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excellent support for mbox-based mailboxes, so there's no reason to
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reinvent the wheel. Some popular mbox servers are: <a
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href="http://www.eudora.com/qpopper/">Qpopper</a>, <a
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reinvent the wheel. Some popular mbox servers are: <a target="_blank"
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href="http://www.eudora.com/qpopper/">Qpopper</a>, <a target="_blank"
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href="http://www.washington.edu/imap/">UW-IMAP</a>, and <a
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href="http://neomail.sourceforge.net/">NeoMail</a>.<br />
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target="_blank" href="http://neomail.sourceforge.net/">NeoMail</a>.</li>
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<li>A faxmail gateway (experimental) that forwards E-mail messages via fax
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(requires a compatible class 2 faxmodem). Courier doesn't implement the
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actual faxing all by itself, actually. Courier uses additional software
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(which must be separately installed), to take care of the low-level
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details. The popular <a
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details. The popular <a target="_blank"
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href="http://alpha.greenie.net/mgetty/">mgetty+sendfax</a> package talks
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to the faxmodem and handles the actual faxing. Conversion of E-mail
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messages to fax pages is done by <a
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messages to fax pages is done by <a target="_blank"
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href="http://www.ghostscript.com/">ghostscript</a>, troff or <a
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href="http://www.gnu.org/software/groff/groff.html">groff</a>, and the <a
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href="http://netpbm.sourceforge.net/">NetPBM</a> library. Courier glues
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all of these pieces together in a seamless manner any time an E-mail
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message addressed to <tt><phonenumber@fax></tt> is received. The
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main textual body of the E-mail message is placed on a cover page, and
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any attachments are converted to fax image format and transmitted after
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the cover page. At this time, Courier knows how to send plain text, PDF,
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and Postscript attachments. GIF, JPEG, and PNG images can be sent to (one
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image per page).<br />
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The additional software packages that were mentioned previously are
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usually already included in most Linux and BSD installations. In most
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cases no additional software really needs to be installed in order to get
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faxmailing up and running.<br />
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target="_blank" href="http://netpbm.sourceforge.net/">NetPBM</a> library.
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Courier glues all of these pieces together in a seamless manner any time
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an E-mail message addressed to <tt><phonenumber@fax></tt> is
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received. The main textual body of the E-mail message is placed on a
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cover page, and any attachments are converted to fax image format and
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transmitted after the cover page. At this time, Courier knows how to send
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plain text, PDF, and Postscript attachments. GIF, JPEG, and PNG images
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can be sent to (one image per page). The additional software packages
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that were mentioned previously are usually already included in most Linux
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and BSD installations. In most cases no additional software really needs
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to be installed in order to get faxmailing up and running.</li>
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<li>Courier includes a mailing list manager, with fully automatic bounce
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<li>You don't need a full-blown mail server? Courier's <a
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href="http://www.inter7.com/courierimap/">IMAP server</a>, <a
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href="http://www.inter7.com/sqwebmail/">webmail server</a>, and <a
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href="http://www.flounder.net/~mrsam/maildrop/">mail filter</a> are
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available as independent packages that can be used with other mail
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servers (as long as the other mail servers store mail in <a
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<li>You don't need a full-blown mail server? Courier's <a target="_top"
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href="http://www.courier-mta.org/courierimap/">IMAP server</a>, <a
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target="_top" href="http://www.courier-mta.org/sqwebmail/">webmail
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server</a>, and <a target="_top"
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href="http://www.courier-mta.org/maildrop/">mail filter</a> are available
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as independent packages that can be used with other mail servers (as long
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as the other mail servers store mail in <a target="_blank"
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href="http://www.qmail.org/man/man5/maildir.html">maildirs</a>). These
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sub-packages are assembled from the same source code tree. The only
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difference is the top level makefile. Note: the independent builds are
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not always in sync with the main Courier build at any given time. They
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follow their own schedule, and may include a slightly older, or even more
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recent, code base! Over time, however, everything always syncs together
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since all builds are assembled from the same source code repository.<br />
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since all builds are assembled from the same source code repository.</li>
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<li>SOCKSv5 support. Courier can punch through a SOCKS firewall to send
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outgoing mail. Receiving mail through a SOCKS firewall is not yet
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supported, unfortunately. To use SOCKS you need to install <!-- either <a
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href="http://www.inet.no/dante/">Dante</a>, or -->
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NEC's <a href="http://www.socks.nec.com/socks5.html">SOCKSv5</a>
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supported. To use SOCKS you need to install Courier's Socks 5 proxy
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<li>PAM, LDAP, PostgreSQL (beta), or MySQL authentication. LDAP
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authentication requires <a href="http://www.openldap.org">OpenLDAP</a> to
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be installed. LDAP-based mail routing is also supported.<br />
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authentication requires <a target="_blank"
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href="http://www.openldap.org">OpenLDAP</a> to be installed. LDAP-based
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mail routing is also supported.</li>
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<li>Gateway mail to/from UUCP (if compatible UUCP software is separately
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<li>Authenticated SMTP.<br />
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<li><code>XVERP</code> and <code>XEXDATA</code> ESMTP extensions.<br />
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<li>Authenticated SMTP.</li>
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<li><code>XVERP</code> and <code>XEXDATA</code> ESMTP extensions.</li>
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<li>DNS-based blacklists. Ability to exempt whitelisted IP addresses from
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the blacklists.<br />
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<li>Integrated mail filtering. An API is provided for installing arbitrary
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external mail filters, and the system administrator can selectively
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enable for any mail source (ESMTP, UUCP, locally submitted mail) for
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integral part of the mail server. Unwanted mail is rejected, and is not
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accepted by Courier for delivery (the external mail relay receives the
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error, and it becomes the external relay's problem as to what to do with
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unwanted junk mail).<br />
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unwanted junk mail).</li>
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<li>Partial ability to import <code>sendmail</code>'s aliases file, but not
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all aspects of <code>sendmail</code>'s aliasing is supported - like
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delivering to programs, for example. Still, most simple
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<code>aliases</code> files should be usable.<br />
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<code>aliases</code> files should be usable.</li>
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<li>Optional ability to import most of <code>Qmail</code>'s
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<code>.qmail</code> files (Courier uses an almost 100% compatible local
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mail delivery instruction format).<br />
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mail delivery instruction format).</li>
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<li>Most major components of Courier can be installed in non-default
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directories, allowing extreme customization for your particular
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<li>You can set a maximum number of messages to deliver simultaneously to
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the same host. This, in fact, is strongly encouraged so that a single
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nonfunctioning domain does not take up all available delivery slots. Rate
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limiting is implemented in the main scheduler, and applies to any
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transport mechanism, not just ESMTP.<br />
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transport mechanism, not just ESMTP.</li>
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<li>Mailing list administrators can specify a backup relay and have mail
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that's not immediately deliverable offloaded to a backup server (this
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feature needs testing/feedback).</li>
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Message scheduling, dispatching, and the actual transport mechanism are
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completely modularized. Different message transport mechanisms such as UUCP
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can be implemented in a simple plug-in fashion, however some C coding will be
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<h2>Message scheduling</h2>
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Courier supports VERPs, multiple recipients per message, and
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RFC1894-compliant delivery status notifications.
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RFC1894-compliant delivery status notifications.
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<h2>Load limiting</h2>
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You can set a maximum number of messages to deliver simultaneously to the
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same host. This, in fact, is strongly encouraged so that a single
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nonfunctioning domain does not take up all available delivery slots. Rate
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limiting is implemented in the main scheduler, and applies to any transport
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mechanism, not just ESMTP.
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mechanism, not just ESMTP.
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<h2>Automatic restarts and garbage cleanup</h2>
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Courier's scheduling engine restarts itself automatically, on a regular
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basis. This helps with memory fragmentation. Courier tries to restart itself
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during periods of system inactivity.
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during periods of system inactivity.
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<h2>Smart organization of the message queue and temporary directories</h2>
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Courier automatically creates subdirectories when necessary, and deletes them
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