2013-03-13 The POP3 Extension Mechanism updates to define a mechanism to announce support for optional commands, extensions, and unconditional server behavior. Please see Section 1 of for details regarding POP3 Extensions. Standards Track or IESG-approved Experimental RFC TOP none TOP none both no TRANSACTION USER none USER, PASS none both no AUTHENTICATION SASL mech list AUTH none both no AUTHENTICATION RESP-CODES none none none both no n/a LOGIN-DELAY secs, USER none USER, PASS, APOP, AUTH both yes n/a PIPELINING none none all both no n/a EXPIRE days, NEVER, USER none none both yes n/a UIDL none UIDL none both no TRANSACTION IMPLEMENTATION text none none TRANS, both no n/a STLS none STLS USER, PASS, CAPA both no AUTHENTICATION AUTH-RESP-CODE none none none both no n/a UTF8 USER UTF8 USER, PASS, APOP, LIST, TOP, RETR both no AUTHORIZATION LANG none LANG all both no AUTHORIZATION TRANSACTION RFC Specification Required See Section 9 of for details regarding Response Codes. LOGIN-DELAY -ERR USER, PASS, APOP, AUTH IN-USE -ERR PASS, APOP, AUTH SYS/PERM -ERR All commands SYS/TEMP -ERR All commands AUTH -ERR Any authentication command including AUTH USER , PASS, or APOP. UTF8 -ERR LIST, TOP, RETR see spec for details Returning the AUTH response code to the USER command reveals to the client that the specified user exists. It is strongly RECOMMENDED that the server not issue this response code to the USER command.