&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;its&nbsp;relay&nbsp;host&nbsp;and&nbsp;sends&nbsp;all&nbsp;mail&nbsp;to&nbsp;it.&nbsp;&nbsp;This&nbsp;relay&nbsp;host&nbsp;could&nbsp;<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;be,&nbsp;but&nbsp;need&nbsp;not&nbsp;be,&nbsp;the&nbsp;POP3&nbsp;server&nbsp;host&nbsp;for&nbsp;the&nbsp;client&nbsp;host.&nbsp;&nbsp;Of&nbsp;<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;course,&nbsp;the&nbsp;relay&nbsp;host&nbsp;must&nbsp;accept&nbsp;mail&nbsp;for&nbsp;delivery&nbsp;to&nbsp;arbitrary&nbsp;<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;recipient&nbsp;addresses,&nbsp;that&nbsp;functionality&nbsp;is&nbsp;not&nbsp;required&nbsp;of&nbsp;all&nbsp;<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;SMTP&nbsp;servers.&nbsp;<br />Myers&nbsp;&amp;&nbsp;Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Standards&nbsp;Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[Page&nbsp;2]&nbsp;<br />&nbsp;<br />RFC&nbsp;1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;May&nbsp;1996&nbsp;<br />3.&nbsp;Basic&nbsp;Operation&nbsp;<br />&nbsp;&nbsp;&nbsp;Initially,&nbsp;the&nbsp;server&nbsp;host&nbsp;starts&nbsp;the&nbsp;POP3&nbsp;service&nbsp;by&nbsp;listening&nbsp;on&nbsp;<br />&nbsp;&nbsp;&nbsp;TCP&nbsp;port&nbsp;110.&nbsp;&nbsp;When&nbsp;a&nbsp;client&nbsp;host&nbsp;wishes&nbsp;to&nbsp;make&nbsp;use&nbsp;of&nbsp;the&nbsp;service,&nbsp;<br />&nbsp;&nbsp;&nbsp;it&nbsp;establishes&nbsp;a&nbsp;TCP&nbsp;connection&nbsp;with&nbsp;the&nbsp;server&nbsp;host.&nbsp;&nbsp;When&nbsp;the&nbsp;<br />&nbsp;&nbsp;&nbsp;connection&nbsp;is&nbsp;established,&nbsp;the&nbsp;POP3&nbsp;server&nbsp;sends&nbsp;a&nbsp;greeting.&nbsp;&nbsp;The&nbsp;<br />&nbsp;&nbsp;&nbsp;client&nbsp;and&nbsp;POP3&nbsp;server&nbsp;then&nbsp;exchange&nbsp;commands&nbsp;and&nbsp;responses&nbsp;<br />&nbsp;&nbsp;&nbsp;(respectively)&nbsp;until&nbsp;the&nbsp;connection&nbsp;is&nbsp;closed&nbsp;or&nbsp;aborted.&nbsp;<br />&nbsp;&nbsp;&nbsp;Commands&nbsp;in&nbsp;the&nbsp;POP3&nbsp;consist&nbsp;of&nbsp;a&nbsp;case-insensitive&nbsp;keyword,&nbsp;possibly&nbsp;<br />&nbsp;&nbsp;&nbsp;followed&nbsp;by&nbsp;one&nbsp;or&nbsp;more&nbsp;arguments.&nbsp;&nbsp;All&nbsp;commands&nbsp;are&nbsp;terminated&nbsp;by&nbsp;a&nbsp;<br />&nbsp;&nbsp;&nbsp;CRLF&nbsp;pair.&nbsp;&nbsp;Keywords&nbsp;and&nbsp;arguments&nbsp;consist&nbsp;of&nbsp;printable&nbsp;ASCII&nbsp;<br />&nbsp;&nbsp;&nbsp;characters.&nbsp;&nbsp;Keywords&nbsp;and&nbsp;arguments&nbsp;are&nbsp;each&nbsp;separated&nbsp;by&nbsp;a&nbsp;single&nbsp;<br />&nbsp;&nbsp;&nbsp;SPACE&nbsp;character.&nbsp;&nbsp;Keywords&nbsp;are&nbsp;three&nbsp;or&nbsp;four&nbsp;characters&nbsp;long.&nbsp;Each&nbsp;<br />&nbsp;&nbsp;&nbsp;argument&nbsp;may&nbsp;be&nbsp;up&nbsp;to&nbsp;40&nbsp;characters&nbsp;long.&nbsp;<br />&nbsp;&nbsp;&nbsp;Responses&nbsp;in&nbsp;the&nbsp;POP3&nbsp;consist&nbsp;of&nbsp;a&nbsp;status&nbsp;indicator&nbsp;and&nbsp;a&nbsp;keyword&nbsp;<br />&nbsp;&nbsp;&nbsp;possibly&nbsp;followed&nbsp;by&nbsp;additional&nbsp;information.&nbsp;&nbsp;All&nbsp;responses&nbsp;are&nbsp;<br />&nbsp;&nbsp;&nbsp;terminated&nbsp;by&nbsp;a&nbsp;CRLF&nbsp;pair.&nbsp;&nbsp;Responses&nbsp;may&nbsp;be&nbsp;up&nbsp;to&nbsp;512&nbsp;characters&nbsp;<br />&nbsp;&nbsp;&nbsp;long,&nbsp;including&nbsp;the&nbsp;terminating&nbsp;CRLF.&nbsp;&nbsp;There&nbsp;are&nbsp;currently&nbsp;two&nbsp;status&nbsp;<br />&nbsp;&nbsp;&nbsp;indicators:&nbsp;positive&nbsp;(&quot;+OK&quot;)&nbsp;and&nbsp;negative&nbsp;(&quot;-ERR&quot;).&nbsp;&nbsp;Servers&nbsp;MUST&nbsp;<br />&nbsp;&nbsp;&nbsp;send&nbsp;the&nbsp;&quot;+OK&quot;&nbsp;and&nbsp;&quot;-ERR&quot;&nbsp;in&nbsp;upper&nbsp;case.&nbsp;<br />&nbsp;&nbsp;&nbsp;Responses&nbsp;to&nbsp;certain&nbsp;commands&nbsp;are&nbsp;multi-line.&nbsp;&nbsp;In&nbsp;these&nbsp;cases,&nbsp;which&nbsp;<br />&nbsp;&nbsp;&nbsp;are&nbsp;clearly&nbsp;indicated&nbsp;below,&nbsp;after&nbsp;sending&nbsp;the&nbsp;first&nbsp;line&nbsp;of&nbsp;the&nbsp;<br />&nbsp;&nbsp;&nbsp;response&nbsp;and&nbsp;a&nbsp;CRLF,&nbsp;any&nbsp;additional&nbsp;lines&nbsp;are&nbsp;sent,&nbsp;each&nbsp;terminated&nbsp;<br />&nbsp;&nbsp;&nbsp;by&nbsp;a&nbsp;CRLF&nbsp;pair.&nbsp;&nbsp;When&nbsp;all&nbsp;lines&nbsp;of&nbsp;the&nbsp;response&nbsp;have&nbsp;been&nbsp;sent,&nbsp;a&nbsp;<br />&nbsp;&nbsp;&nbsp;final&nbsp;line&nbsp;is&nbsp;sent,&nbsp;consisting&nbsp;of&nbsp;a&nbsp;termination&nbsp;octet&nbsp;(decimal&nbsp;code&nbsp;<br />&nbsp;&nbsp;&nbsp;046,&nbsp;&quot;.&quot;)&nbsp;and&nbsp;a&nbsp;CRLF&nbsp;pair.&nbsp;&nbsp;If&nbsp;any&nbsp;line&nbsp;of&nbsp;the&nbsp;multi-line&nbsp;response&nbsp;<br />&nbsp;&nbsp;&nbsp;begins&nbsp;with&nbsp;the&nbsp;termination&nbsp;octet,&nbsp;the&nbsp;line&nbsp;is&nbsp;&quot;byte-stuffed&quot;&nbsp;by&nbsp;<br />&nbsp;&nbsp;&nbsp;pre-pending&nbsp;the&nbsp;termination&nbsp;octet&nbsp;to&nbsp;that&nbsp;line&nbsp;of&nbsp;the&nbsp;response.&nbsp;<br />&nbsp;&nbsp;&nbsp;Hence&nbsp;a&nbsp;multi-line&nbsp;response&nbsp;is&nbsp;terminated&nbsp;with&nbsp;the&nbsp;five&nbsp;octets&nbsp;<br />&nbsp;&nbsp;&nbsp;&quot;CRLF.CRLF&quot;.&nbsp;&nbsp;When&nbsp;examining&nbsp;a&nbsp;multi-line&nbsp;response,&nbsp;the&nbsp;client&nbsp;checks&nbsp;<br />&nbsp;&nbsp;&nbsp;to&nbsp;see&nbsp;if&nbsp;the&nbsp;line&nbsp;begins&nbsp;with&nbsp;the&nbsp;termination&nbsp;octet.&nbsp;&nbsp;If&nbsp;so&nbsp;and&nbsp;if&nbsp;<br />&nbsp;&nbsp;&nbsp;octets&nbsp;other&nbsp;than&nbsp;CRLF&nbsp;follow,&nbsp;the&nbsp;first&nbsp;octet&nbsp;of&nbsp;the&nbsp;line&nbsp;(the&nbsp;<br />&nbsp;&nbsp;&nbsp;termination&nbsp;octet)&nbsp;is&nbsp;stripped&nbsp;away.&nbsp;&nbsp;If&nbsp;so&nbsp;and&nbsp;if&nbsp;CRLF&nbsp;immediately&nbsp;<br />&nbsp;&nbsp;&nbsp;follows&nbsp;the&nbsp;termination&nbsp;character,&nbsp;then&nbsp;the&nbsp;response&nbsp;from&nbsp;the&nbsp;POP&nbsp;<br />&nbsp;&nbsp;&nbsp;server&nbsp;is&nbsp;ended&nbsp;and&nbsp;the&nbsp;line&nbsp;containing&nbsp;&quot;.CRLF&quot;&nbsp;is&nbsp;not&nbsp;considered&nbsp;<br />&nbsp;&nbsp;&nbsp;part&nbsp;of&nbsp;the&nbsp;multi-line&nbsp;response.&nbsp;<br />&nbsp;&nbsp;&nbsp;A&nbsp;POP3&nbsp;session&nbsp;progresses&nbsp;through&nbsp;a&nbsp;number&nbsp;of&nbsp;states&nbsp;during&nbsp;its&nbsp;<br />&nbsp;&nbsp;&nbsp;lifetime.&nbsp;&nbsp;Once&nbsp;the&nbsp;TCP&nbsp;connection&nbsp;has&nbsp;been&nbsp;opened&nbsp;and&nbsp;the&nbsp;POP3&nbsp;<br />&nbsp;&nbsp;&nbsp;server&nbsp;has&nbsp;sent&nbsp;the&nbsp;greeting,&nbsp;the&nbsp;session&nbsp;enters&nbsp;the&nbsp;AUTHORIZATION&nbsp;<br />&nbsp;&nbsp;&nbsp;state.&nbsp;&nbsp;In&nbsp;this&nbsp;state,&nbsp;the&nbsp;client&nbsp;must&nbsp;identify&nbsp;itself&nbsp;to&nbsp;the&nbsp;POP3&nbsp;<br />&nbsp;&nbsp;&nbsp;server.&nbsp;&nbsp;Once&nbsp;the&nbsp;client&nbsp;has&nbsp;successfully&nbsp;done&nbsp;this,&nbsp;the&nbsp;server&nbsp;<br />&nbsp;&nbsp;&nbsp;acquires&nbsp;resources&nbsp;associated&nbsp;with&nbsp;the&nbsp;client's&nbsp;maildrop,&nbsp;and&nbsp;the&nbsp;<br />&nbsp;&nbsp;&nbsp;session&nbsp;enters&nbsp;the&nbsp;TRANSACTION&nbsp;state.&nbsp;&nbsp;In&nbsp;this&nbsp;state,&nbsp;the&nbsp;client&nbsp;<br />&nbsp;&nbsp;&nbsp;requests&nbsp;actions&nbsp;on&nbsp;the&nbsp;part&nbsp;of&nbsp;the&nbsp;POP3&nbsp;server.&nbsp;&nbsp;When&nbsp;the&nbsp;client&nbsp;has&nbsp;<br />Myers&nbsp;&amp;&nbsp;Rose&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Standards&nbsp;Track&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[Page&nbsp;3]&nbsp;<br />&nbsp;<br />RFC&nbsp;1939&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;POP3&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;May&nbsp;1996&nbsp;<br />&nbsp;&nbsp;&nbsp;issued&nbsp;the&nbsp;QUIT&nbsp;command,&nbsp;the&nbsp;session&nbsp;enters&nbsp;the&nbsp;UPDATE&nbsp;state.&nbsp;&nbsp;In&nbsp;<br />&nbsp;&nbsp;&nbsp;this&nbsp;state,&nbsp;the&nbsp;POP3&nbsp;server&nbsp;releases&nbsp;any&nbsp;resources&nbsp;acquired&nbsp;during&nbsp;<br />&nbsp;&nbsp;&nbsp;the&nbsp;TRANSACTION&nbsp;state&nbsp;and&nbsp;says&nbsp;goodbye.&nbsp;&nbsp;The&nbsp;TCP&nbsp;connection&nbsp;is&nbsp;then&nbsp;<br />&nbsp;&nbsp;&nbsp;closed.&nbsp;<br />&nbsp;&nbsp;&nbsp;A&nbsp;server&nbsp;MUST&nbsp;respond&nbsp;to&nbsp;an&nbsp;unrecognized,&nbsp;unimplemented,&nbsp;or&nbsp;<br />&nbsp;&nbsp;&nbsp;syntactically&nbsp;invalid&nbsp;command&nbsp;by&nbsp;responding&nbsp;with&nbsp;a&nbsp;negative&nbsp;status&nbsp;<br />&nbsp;&nbsp;&nbsp;indicator.&nbsp;&nbsp;A&nbsp;server&nbsp;MUST&nbsp;respond&nbsp;to&nbsp;a&nbsp;command&nbsp;issued&nbsp;when&nbsp;the&nbsp;<br />&nbsp;&nbsp;&nbsp;session&nbsp;is&nbsp;in&nbsp;an&nbsp;incorrect&nbsp;state&nbsp;by&nbsp;responding&nbsp;with&nbsp;a&nbsp;negative&nbsp;status&nbsp;<br />&nbsp;&nbsp;&nbsp;indicator.&nbsp;&nbsp;There&nbsp;is&nbsp;no&nbsp;general&nbsp;method&nbsp;for&nbsp;a&nbsp;client&nbsp;to&nbsp;distinguish&nbsp;<br />&nbsp;&nbsp;&nbsp;between&nbsp;a&nbsp;server&nbsp;which&nbsp;does&nbsp;not&nbsp;implement&nbsp;an&nbsp;optional&nbsp;command&nbsp;and&nbsp;a&nbsp;<br />&nbsp;&nbsp;&nbsp;server&nbsp;which&nbsp;is&nbsp;unwilling&nbsp;or&nbsp;unable&nbsp;to&nbsp;process&nbsp;the&nbsp;command.&nbsp;<br />&nbsp;&nbsp;&nbsp;A&nbsp;POP3&nbsp;server&nbsp;MAY&nbsp;have&nbsp;an&nbsp;inactivity&nbsp;autologout&nbsp;timer.&nbsp;&nbsp;Such&nbsp;a&nbsp;timer&nbsp;<br />&nbsp;&nbsp;&nbsp;MUST&nbsp;be&nbsp;of&nbsp;at&nbsp;least&nbsp;10&nbsp;minutes'&nbsp;duration.&nbsp;&nbsp;The&nbsp;receipt&nbsp;of&nbsp;any&nbsp;command&nbsp;<br />&nbsp;&nbsp;&nbsp;from&nbsp;the&nbsp;client&nbsp;during&nbsp;that&nbsp;interval&nbsp;should&nbsp;suffice&nbsp;to&nbsp;reset&nbsp;the&nbsp;<br />&nbsp;&nbsp;&nbsp;autologout&nbsp;timer.&nbsp;&nbsp;When&nbsp;the&nbsp;timer&nbsp;expires,&nbsp;the&nbsp;session&nbsp;does&nbsp;NOT&nbsp;enter&nbsp;<br />&nbsp;&nbsp;&nbsp;the&nbsp;UPDATE&nbsp;state--the&nbsp;server&nbsp;should&nbsp;close&nbsp;the&nbsp;TCP&nbsp;connection&nbsp;without&nbsp;<br />&nbsp;&nbsp;&nbsp;removing&nbsp;any&nbsp;messages&nbsp;or&nbsp;sending&nbsp;any&nbsp;response&nbsp;to&nbsp;the&nbsp;client.&nbsp;<br />4.&nbsp;The&nbsp;AUTHORIZATION&nbsp;State&nbsp;<br />&nbsp;&nbsp;&nbsp;Once&nbsp;the&nbsp;TCP&nbsp;connection&nbsp;has&nbsp;been&nbsp;opened&nbsp;by&nbsp;a&nbsp;POP3&nbsp;client,&nbsp;the&nbsp;POP3&nbsp;<br />&nbsp;&nbsp;&nbsp;server&nbsp;issues&nbsp;a&nbsp;one&nbsp;line&nbsp;greeting.&nbsp;&nbsp;This&nbsp;can&nbsp;be&nbsp;any&nbsp;positive&nbsp;<br />&nbsp;&nbsp;&nbsp;response.&nbsp;&nbsp;An&nbsp;example&nbsp;might&nbsp;be:&nbsp;<br />&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;S:&nbsp;&nbsp;+OK&nbsp;POP3&nbsp;server&nbsp;ready&nbsp;<br />&nbsp;&nbsp;&nbsp;The&nbsp;POP3&nbsp;session&nbsp;is&nbsp;now&nbsp;in&nbsp;the&nbsp;AUTHORIZATION&nbsp;state.&nbsp;&nbsp;The&nbsp;client&nbsp;must&nbsp;<br />&nbsp;&nbsp;&nbsp;now&nbsp;identify&nbsp;and&nbsp;authenticate&nbsp;itself&nbsp;to&nbsp;the&nbsp;POP3&nbsp;server.&nbsp;&nbsp;Two&nbsp;<br />&nbsp;&nbsp;&nbsp;possible&nbsp;mechanisms&nbsp;for&nbsp;doing&nbsp;this&nbsp;are&nbsp;described&nbsp;in&nbsp;this&nbsp;document,&nbsp;<br />&nbsp;&nbsp;&nbsp;the&nbsp;USER&nbsp;and&nbsp;PASS&nbsp;command&nbsp;combination&nbsp;and&nbsp;the&nbsp;APOP&nbsp;command.&nbsp;&nbsp;Both&nbsp;<br />&nbsp;&nbsp;&nbsp;mechanisms&nbsp;are&nbsp;described&nbsp;later&nbsp;in&nbsp;this&nbsp;document.&nbsp;&nbsp;Additional&nbsp;<br />&nbsp;&nbsp;&nbsp;authentication&nbsp;mechanisms&nbsp;are&nbsp;described&nbsp;in&nbsp;[RFC1734].&nbsp;&nbsp;While&nbsp;there&nbsp;is&nbsp;<br />&nbsp;&nbsp;&nbsp;no&nbsp;single&nbsp;authentication&nbsp;mechanism&nbsp;that&nbsp;is&nbsp;required&nbsp;of&nbsp;all&nbsp;POP3&nbsp;<br />&nbsp;&nbsp;&nbsp;servers,&nbsp;a&nbsp;POP3&nbsp;server&nbsp;must&nbsp;of&nbsp;course&nbsp;support&nbsp;at&nbsp;least&nbsp;one&nbsp;<br />&nbsp;&nbsp;&nbsp;authentication&nbsp;mechanism.&nbsp;<br />&nbsp;&nbsp;&nbsp;Once&nbsp;the&nbsp;POP3&nbsp;server&nbsp;has&nbsp;determined&nbsp;through&nbsp;the&nbsp;use&nbsp;of&nbsp;any&nbsp;<br />&nbsp;&nbsp;&nbsp;authentication&nbsp;command&nbsp;that&nbsp;the&nbsp;client&nbsp;should&nbsp;be&nbsp;given&nbsp;access&nbsp;to&nbsp;the&nbsp;<br />&nbsp;&nbsp;&nbsp;appropriate&nbsp;maildrop,&nbsp;the&nbsp;POP3&nbsp;server&nbsp;then&nbsp;acquires&nbsp;an&nbsp;exclusive-&nbsp;<br />&nbsp;&nbsp;&nbsp;access&nbsp;lock&nbsp;on&nbsp;the&nbsp;maildrop,&nbsp;as&nbsp;necessary&nbsp;to&nbsp;prevent&nbsp;messages&nbsp;from&nbsp;<br />&nbsp;&nbsp;&nbsp;being&nbsp;modified&nbsp;or&nbsp;removed&nbsp;before&nbsp;the&nbsp;session&nbsp;enters&nbsp;the&nbsp;UPDATE&nbsp;state.&nbsp;<br />&nbsp;&nbsp;&nbsp;If&nbsp;the&nbsp;lock&nbsp;is&nbsp;successfully&nbsp;acquired,&nbsp;the&nbsp;POP3&nbsp;server&nbsp;responds&nbsp;with&nbsp;a&nbsp;<br />&nbsp;&nbsp;&nbsp;positive&nbsp;status&nbsp;indicator.&nbsp;&nbsp;The&nbsp;POP3&nbsp;session&nbsp;now&nbsp;enters&nbsp;the&nbsp;<br />&nbsp;&nbsp;&nbsp;TRANSACTION&nbsp;state,&nbsp;with&nbsp;no&nbsp;messages&nbsp;marked&nbsp;as&nbsp;deleted.&nbsp;&nbsp;If&nbsp;the&nbsp;<br />