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<section id="known-protocol-limitations">
<h1>Known Protocol Limitations<a class="headerlink" href="#known-protocol-limitations" title="Permalink to this heading"></a></h1>
<p>This chapter lists known limitations to protocols commonly in use today, that
may impact your deployment.</p>
<section id="pop3-and-mailbox-locking">
<h2>POP3 and Mailbox Locking<a class="headerlink" href="#pop3-and-mailbox-locking" title="Permalink to this heading"></a></h2>
<p>POP3, as described in <span class="target" id="index-0"></span><a class="rfc reference external" href="https://datatracker.ietf.org/doc/html/rfc1939.html"><strong>RFC 1939</strong></a>, requires a mailbox to be locked by a POP3
session.</p>
<p>As such, when POP3 is used simultaneously across multiple clients, and a common
set of mailboxes, an error similar to the following would occur:</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">Mailbox</span> <span class="n">locked</span> <span class="n">by</span> <span class="n">POP</span> <span class="n">server</span><span class="o">.</span>
</pre></div>
</div>
<p>The exact error message may be subject to the specific error message a client
application wishes to display.</p>
</section>
<section id="cyrus-imap-pop3-implementation">
<h2>Cyrus IMAP POP3 Implementation<a class="headerlink" href="#cyrus-imap-pop3-implementation" title="Permalink to this heading"></a></h2>
<p>The Cyrus IMAP POP3 server implementation does not have the aforementioned
problem of POP3 sessions locking mailboxes. As of version 2.4.0, Cyrus IMAP
allows multiple POP3 sessions to operate on a single mailbox by providing a
<em>virtual</em> snapshot of the mailbox, and all operations are executed to this
snapshot. A safety mechanism ensures no messages are deleted until after all
existing operations have closed the mailbox - including IMAP, LMTP and POP.</p>
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