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 <article class="doc">
 <h1 class="page">Post Office Protocol (POP)</h1>
 <div class="sect1">
 <h2 id="_overview"><a class="anchor" href="#_overview"></a>Overview</h2>
 <div class="sectionbody">
 <div class="paragraph">
 <p>The <a href="https://en.wikipedia.org/wiki/Post_Office_Protocol">Post Office Protocol</a>,
 or POP, is described in the original specification
 as being "a simple method for workstations to dynamically access mail from
 a <a href="../storage/index.html" class="page">mailbox server</a>" (though interestingly "mailbox server"
 is never actually defined in this specification nor in later specifications. 🤔)</p>
 </div>
 <div class="paragraph">
 <p>POP was proposed as the first non-<a href="smtp.html" class="page">SMTP</a> protocol aimed
 at users for the purpose of retrieving emails stored remotely for local consumption.
 POP was developed in pre-spam days, so was inherently a non-secure protocol.
 It has been updated several times to address several issues, including lack of
 security, so that POP3 is now the current specification.</p>
 </div>
 <div class="paragraph">
 <p>As the name suggests, the protocol is modeled after a post office. The analogy being:</p>
 </div>
 <div class="ulist">
 <ul>
 <li>
 <p>The various SMTP servers along the transmission chain are like post offices</p>
 </li>
 <li>
 <p>The user&#8217;s mail server is the local post office that will dispatch the mail to the recipient</p>
 </li>
 <li>
 <p>The postman is, ummm, well no analogy is perfect 😬</p>
 </li>
 <li>
 <p>The mail gets dropped into the user&#8217;s mailbox (though just to make the analogy even
 muddier there is an option to leave a copy on the server)</p>
 </li>
 </ul>
 </div>
 <div class="paragraph">
 <p>Analogies aside, the protocol worked quite well for most users, and is still widely
 in use today.</p>
 </div>
 <div class="paragraph">
 <p>POP requires essentially two pieces, the POP Client and the POP Server. How the
 message gets initially delivered to the POP Server and how a message is managed
 once delivered to the POP Client is out of scope of the POP protocol.</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="_pop"><a class="anchor" href="#_pop"></a>POP</h3>
 <div class="paragraph">
 <p>POP has been around since the early days of email. The original specification
 <a href="#_rfc918">RFC918</a> was published in 1984.</p>
 </div>
 <div class="sect3">
 <h4 id="_rfc918"><a class="anchor" href="#_rfc918"></a>RFC918</h4>
 <div class="paragraph">
 <p><a href="https://tools.ietf.org/html/rfc918">RFC918</a> ("Post Office Protocol") was the original
 specification published in 1984. The RFC "suggests a simple method for workstations
 to dynamically access mail from a mailbox server."
 Like <a href="smtp.html" class="page">SMTP</a> that came before it, POP is a session-based
 protocol with a handful of defined commands.
 This RFC was obsoleted shortly after its publication by <a href="#_rfc937">RFC937</a>.</p>
 </div>
 </div>
 </div>
 <div class="sect2">
 <h3 id="_pop2"><a class="anchor" href="#_pop2"></a>POP2</h3>
 <div class="sect3">
 <h4 id="_rfc937"><a class="anchor" href="#_rfc937"></a>RFC937</h4>
 <div class="paragraph">
 <p><a href="https://tools.ietf.org/html/rfc937">RFC937</a> ("Post Office Protocol - Version 2") or more
 commonly "POP2" was a revision of the original <a href="#_rfc918">RFC918</a> specification. POP2 specifies
 connection over port 109, and clarifies that "it is important for the mailbox to be on
 an 'always up' machine and that a workstation may be frequently powered down, or
 otherwise unavailable as an SMTP server." In other words, as the Internet continued to
 evolve, this specification was an attempt to evolve with it.</p>
 </div>
 <div class="paragraph">
 <p>POP2 was replaced by POP3, which was first published as <a href="#_rfc1081">RFC1081</a>.</p>
 </div>
 </div>
 </div>
 <div class="sect2">
 <h3 id="_pop3"><a class="anchor" href="#_pop3"></a>POP3</h3>
 <div class="sect3">
 <h4 id="_rfc1081"><a class="anchor" href="#_rfc1081"></a>RFC1081</h4>
 <div class="paragraph">
 <p><a href="https://tools.ietf.org/html/rfc1081">RFC1081</a> ("Post Office Protocol - Version 3")
 was the first published version of POP3,
 made available in 1988. It adds additional functionality for managing mail, and
 perhaps most importantly, it adds an authentication mechanism. This version was
 obsoleted by <a href="#_rfc1225">RFC1225</a>.</p>
 </div>
 </div>
 <div class="sect3">
 <h4 id="_rfc1225"><a class="anchor" href="#_rfc1225"></a>RFC1225</h4>
 <div class="paragraph">
 <p><a href="https://tools.ietf.org/html/rfc1225">RFC1225</a> ("Post Office Protocol - Version 3")
 obsoleted <a href="#_rfc1081">RFC1081</a>. It is described as being a "republication" of <a href="#_rfc1081">RFC1081</a>,
 and does not appear to add anything significant. It was obsoleted by
 <a href="#_rfc1460">RFC1460</a>.</p>
 </div>
 </div>
 <div class="sect3">
 <h4 id="_rfc1460"><a class="anchor" href="#_rfc1460"></a>RFC1460</h4>
 <div class="paragraph">
 <p><a href="https://tools.ietf.org/html/rfc1460">RFC1460</a> ("Post Office Protocol - Version 3")
 provided some relatively minor updates to <a href="#_rfc1225">RFC1225</a>, and was itself obsoleted
 about one year later by <a href="#_rfc1725">RFC1725</a>.</p>
 </div>
 </div>
 <div class="sect3">
 <h4 id="_rfc1725"><a class="anchor" href="#_rfc1725"></a>RFC1725</h4>
 <div class="paragraph">
 <p><a href="https://tools.ietf.org/html/rfc1725">RFC1725</a> ("Post Office Protocol - Version 3")
 was again a relatively minor update of <a href="#_rfc1460">RFC1460</a>, and was itself obsoleted
 about two years later by <a href="#_rfc1939">RFC1939</a>.</p>
 </div>
 </div>
 <div class="sect3">
 <h4 id="_rfc1939"><a class="anchor" href="#_rfc1939"></a>RFC1939</h4>
 <div class="paragraph">
 <p><a href="https://tools.ietf.org/html/rfc1939">RFC1939</a> ("Post Office Protocol - Version 3")
 is the most recent specification of POP3, and is the version in use today.</p>
 </div>
 <div class="paragraph">
 <p>This specification seems to imply that POP3 is used for "simple" transactions,
 and for more "advanced" transactions <a href="imap.html" class="page">IMAP</a> is available.</p>
 </div>
 </div>
 </div>
 <div class="sect2">
 <h3 id="_pop4"><a class="anchor" href="#_pop4"></a>POP4</h3>
 <div class="paragraph">
 <p>Although there was development on the <a href="http://www.pop4.org/">POP4</a> specification,
 development appears to have stalled for several years, making POP3 the
 still-current protocol.</p>
 </div>
 </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>POP relates to&#8230;&#8203;</p>
 </div>
 <div class="ulist">
 <ul>
 <li>
 <p><strong><a href="imap.html" class="page">IMAP</a></strong> as a "competing" protocol</p>
 </li>
 <li>
 <p><strong><a href="../glossary.html#email_client" class="page">Email clients</a></strong>, in that the client acts as a POP Client</p>
 </li>
 <li>
 <p><strong><a href="../storage/index.html" class="page">Storage</a></strong> in that messages are stored temporarily on
 a ????  while awaiting transfer to the client</p>
 </li>
 <li>
 <p><strong><a href="smtp.html" class="page">SMTP</a></strong>, in that messages will normally be
 delivered to the IMAP Server via SMTP.</p>
 </li>
 </ul>
 </div>
 </div>
 </div>
 </article>
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