Internet Assigned Numbers Authority Post Office Protocol version 3 (POP3) Extension Mechanism Last Updated 2013-03-13 Note The POP3 Extension Mechanism [RFC2449] updates [RFC1939] to define a mechanism to announce support for optional commands, extensions, and unconditional server behavior. Please see Section 1 of [RFC2449] for details regarding POP3 Extensions. Available Formats [IMG] XML [IMG] HTML [IMG] Plain text Registries included below * POP3 Capabilities * POP3 Response Codes POP3 Capabilities Registration Procedure(s) Standards Track or IESG-approved Experimental RFC Reference [RFC2449] Available Formats [IMG] CSV CAPA Tag CAPA Args Added Cmds Cmds Affected List Diffs Cmd Valid References TOP none TOP none both no TRANSACTION [RFC2449][RFC1939] USER none USER, PASS none both no AUTHENTICATION [RFC2449][RFC1939] SASL mech list AUTH none both no AUTHENTICATION [RFC2449][RFC5034] RESP-CODES none none none both no n/a [RFC2449] LOGIN-DELAY secs, USER none USER, PASS, APOP, AUTH both yes n/a [RFC2449] PIPELINING none none all both no n/a [RFC2449] EXPIRE days, NEVER, USER none none both yes n/a [RFC2449] UIDL none UIDL none both no TRANSACTION [RFC2449][RFC1939] IMPLEMENTATION text none none TRANS, both no n/a [RFC2449][RFC1939] STLS none STLS USER, PASS, CAPA both no AUTHENTICATION [RFC2595] AUTH-RESP-CODE none none none both no n/a [RFC3206] UTF8 USER UTF8 USER, PASS, APOP, LIST, TOP, RETR both no AUTHORIZATION [RFC6856] LANG none LANG all both no AUTHORIZATION TRANSACTION [RFC6856] POP3 Response Codes Registration Procedure(s) RFC Specification Required Reference [RFC2449] Note See Section 9 of [RFC2449] for details regarding Response Codes. Available Formats [IMG] CSV Response Code Response Types Commands Reference LOGIN-DELAY -ERR USER[1], PASS, APOP, AUTH [RFC2449] IN-USE -ERR PASS, APOP, AUTH [RFC2449] SYS/PERM -ERR All commands [RFC3206] SYS/TEMP -ERR All commands [RFC3206] AUTH -ERR Any authentication command including AUTH USER [2], PASS, or APOP. [RFC3206] UTF8 -ERR LIST, TOP, RETR [RFC6856] Footnotes [1] see spec for details [2] 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. Licensing Terms