Editing Post Office Protocol (section)

==Extensions and specifications==
An extension mechanism was proposed in RFC 2449 to accommodate general extensions as well as announce in an organized manner support for optional commands, such as TOP and UIDL. The RFC did not intend to encourage extensions, and reaffirmed that the role of POP3 is to provide simple support for mainly download-and-delete requirements of mailbox handling.

The extensions are termed capabilities and are listed by the CAPA command. With the exception of APOP, the optional commands were included in the initial set of capabilities. Following the lead of ESMTP (RFC 5321), capabilities beginning with an X signify local capabilities.

===STARTTLS===
The STARTTLS extension allows the use of [[Transport Layer Security]] (TLS) or [[Secure Sockets Layer]] (SSL) to be negotiated using the ''STLS'' command, on the standard POP3 port, rather than an alternate. Some clients and servers instead use the alternate-port method, which uses TCP port 995 (POP3S).

===SDPS===
[[Demon Internet]] introduced extensions to POP3 that allow multiple accounts per domain, and has become known as ''Standard Dial-up POP3 Service'' (SDPS).<ref>[http://e.demon.net/helpdesk/producthelp/mail/sdps-tech.html/ Demon Online Help Centre] {{webarchive|url=https://archive.today/20110723084318/http://e.demon.net/helpdesk/producthelp/mail/sdps-tech.html/ |date=2011-07-23 }}. E.demon.net (2013-01-23). Retrieved on 2013-07-17.</ref> To access each account, the username includes the hostname, as ''john@hostname'' or ''john+hostname''.

Google Apps uses the same method.<ref>{{cite web|url=http://mail.google.com/support/bin/answer.py?answer=34383|title=Google Apps and Gmail differences|website=Gmail Help Center|archive-url=https://web.archive.org/web/20080913155355/http://mail.google.com/support/bin/answer.py?answer=34383|archive-date=September 13, 2008}}</ref>

===Kerberized Post Office Protocol===
In [[computing]], local [[e-mail client]]s can use the '''Kerberized Post Office Protocol''' ('''KPOP'''), an [[application layer|application-layer]] [[Internet standard]] [[protocol (computing)|protocol]], to retrieve [[e-mail]] from a remote [[mail server|server]] over a [[Internet protocol suite|TCP/IP]] connection. The KPOP protocol is based on the POP3 protocol – differing in that it adds [[Kerberos (protocol)|Kerberos]] security and that it runs by default over [[Transmission Control Protocol|TCP]] port number 1109 instead of 110. One mail server software implementation is found in the [[Cyrus IMAP server]].