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<li><a href="../../index.html">Apache James Server</a></li>
<li><a href="../index.html">Concepts</a></li>
<li><a href="index.html">Storage</a></li>
<li><a href="mailbox.html">Mailboxes</a></li>
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<div class="edit-this-page"><a href="https://github.com/apache/james-project/blob/master/docs/modules/concepts/pages/storage/mailbox.adoc">Edit this Page</a></div>
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<article class="doc">
<h1 class="page">Mailboxes</h1>
<div class="sect1">
<h2 id="_overview"><a class="anchor" href="#_overview"></a>Overview</h2>
<div class="sectionbody">
<div class="paragraph">
<p>To (over)use the analogy once again of a postal system, we think of a mailbox
as a place where a letter is placed to await pickup by its recipient. Although
current methods have evolved considerably, the original idea used for the development
of email was exactly that. The concept <a href="#_rfc196">has been around officially</a> since
at least 1971. The image of a mailbox is so prevalent that, although current
systems have evolved to do so much more than just hold messages for processing,
the industry continues to apply the concept.</p>
</div>
<div class="paragraph">
<p>The more general term is "Email Storage". It is necessary to store email for
many reasons:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>As a mailbox system &mdash; part of a queue to await processing</p>
</li>
<li>
<p>As a persistence mechanism &mdash; a place where you can go back and predictably
locate a message that you may want to refer to again in the future</p>
</li>
<li>
<p>As an archive &mdash; a place where you may want to store "deleted" emails so that
you can potentially retrieve them if you wish to change your mind later</p>
</li>
<li>
<p>As a temporary step in a process &mdash; perhaps you have a "todo" folder where
you temporarily store an email that you intend to process soon</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>There are numerous ways to store emails, just as there are numerous reasons for
storing them:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>A <a href="#_specifications">standardized</a> "mailbox" format</p>
</li>
<li>
<p>A relational database</p>
</li>
<li>
<p>A custom file-based mechanism</p>
</li>
<li>
<p>Etc.</p>
</li>
</ul>
</div>
<div class="paragraph">
<p>This section briefly describes how email storage is a core concept in an
email system.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_specifications"><a class="anchor" href="#_specifications"></a>Specifications</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_electronic_mail_box"><a class="anchor" href="#_electronic_mail_box"></a>Electronic Mail Box</h3>
<div class="sect3">
<h4 id="_rfc196"><a class="anchor" href="#_rfc196"></a>RFC196</h4>
<div class="paragraph">
<p>The concept of an "electronic mail box" was
<a href="https://tools.ietf.org/html/rfc196">initially specified</a> way back in 1971.
It is amusing to read the retro concepts in the initial description of this
document:</p>
</div>
<div class="literalblock">
<div class="content">
<pre>The purpose of this protocol is to provide at each site a
standard mechanism to receive sequential files for immediate or
deferred printing or other uses. The files for deferred printing
would probably be stored on intermediate disk files [...].
It is also assumed that there would be a program at the sending
site [...]. This program could probably be accessed as a subcommand
of the Telnet program.</pre>
</div>
</div>
<div class="paragraph">
<p>This specification was obsoleted by <a href="#_rfc221">RFC221</a>.</p>
</div>
</div>
<div class="sect3">
<h4 id="_rfc221"><a class="anchor" href="#_rfc221"></a>RFC221</h4>
<div class="paragraph">
<p><a href="https://tools.ietf.org/html/rfc221">RFC221</a> ("A Mail Box Protocol, Version-2") was
published in 1971. It added the possibility of using FTP in addition to the
"Data Transfer Protocol" used in <a href="#_rfc196">RFC196</a>. This version was obsoleted by
<a href="#_rfc278">RFC278</a>.</p>
</div>
</div>
<div class="sect3">
<h4 id="_rfc278"><a class="anchor" href="#_rfc278"></a>RFC278</h4>
<div class="paragraph">
<p><a href="https://tools.ietf.org/html/rfc278">RFC278</a> ("Revision of the Mail Box Protocol")
was published in 1971. It provided a number of updates to <a href="#_rfc221">RFC221</a>.</p>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_maildir"><a class="anchor" href="#_maildir"></a>Maildir</h3>
<div class="paragraph">
<p><a href="https://en.wikipedia.org/wiki/Maildir">Maildir</a> is a file-based storage format invented
by <a href="https://en.wikipedia.org/wiki/Daniel_J._Bernstein">Dan Bernstein</a>.
A major design objective was apparently to delegate file locking to the operating system.</p>
</div>
</div>
<div class="sect2">
<h3 id="_mbox"><a class="anchor" href="#_mbox"></a>Mbox</h3>
<div class="paragraph">
<p>The <a href="https://en.wikipedia.org/wiki/Mbox">mbox</a> email storage format was formally defined
by <a href="https://tools.ietf.org/html/rfc4155">RFC4155</a>. It is a formalization of a <em>de facto</em>
format used by UNIX-like operating systems.</p>
</div>
</div>
<div class="sect2">
<h3 id="_mix"><a class="anchor" href="#_mix"></a>MIX</h3>
<div class="paragraph">
<p>The <a href="https://en.wikipedia.org/wiki/MIX_(email)">MIX</a> email storage format was developed by
<a href="https://en.wikipedia.org/wiki/Mark_Crispin">Mark Crispin</a>, the original author of the
<a href="../protocols/imap.html" class="page">IMAP specification</a>. Its
<a href="https://www.mail-archive.com/imap-uw@u.washington.edu/msg00540.html">design goals</a> were:</p>
</div>
<div class="ulist">
<ul>
<li>
<p>greater robustness against corruption caused by hardware or
software failures. Many failures are "self-healing".</p>
</li>
<li>
<p>far fewer risky random-access I/O operations; a single false
pointer calculation in other formats will corrupt the mailbox.</p>
</li>
<li>
<p>greater ease to repair damaged mailboxes.</p>
</li>
<li>
<p>(much) greater performance.</p>
</li>
<li>
<p>extensibility for new IMAP capabilities such as annotations,
conditional store, or more aggressing caching.</p>
</li>
</ul>
</div>
</div>
<div class="sect2">
<h3 id="_rfc5322"><a class="anchor" href="#_rfc5322"></a>RFC5322</h3>
<div class="paragraph">
<p><a href="https://tools.ietf.org/html/rfc5322">RFC5322</a> weighs in on how to describe a "mailbox":</p>
</div>
<div class="literalblock">
<div class="content">
<pre>A mailbox receives mail. It is a 'conceptual entity' that does not necessarily
pertain to file storage. It further exemplifies that some sites may choose to
print mail on a printer and deliver the output to the addressee's desk, much
like a traditional fax transmission.</pre>
</div>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_indexing"><a class="anchor" href="#_indexing"></a>Indexing</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Once a system grows over time it will likely contain many messages
stored in its <a href="#">Mailboxes</a>.
Searching for the right email becomes increasingly difficult. At some point
it becomes useful to use an <a href="https://en.wikipedia.org/wiki/Search_engine_indexing">indexing</a></p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_related_concepts"><a class="anchor" href="#_related_concepts"></a>Related Concepts</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Email Storage is related to&#8230;&#8203;</p>
</div>
<div class="ulist">
<ul>
<li>
<p><strong><a href="../protocols/pop.html" class="page">POP</a></strong>, as the POP protocol mandates interaction
with a user&#8217;s "mailbox"</p>
</li>
<li>
<p><strong><a href="../protocols/imap.html" class="page">IMAP</a></strong>, as the IMAP protocol is all about storing
messages on an IMAP Server</p>
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<p><strong><a href="../glossary.html#email_client" class="page">Email clients</a></strong>, as the client will store mail locally, usually in the form
of a "mailbox"</p>
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<p><strong><a href="../protocols/smtp.html" class="page">SMTP</a></strong> as the protocol is related to transmitting
messages from one mailbox to another</p>
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