What does the POP3 CAPA UIDL command do?

Question

What does the POP3 CAPA UIDL command do?

Answer 1

CAPA is one command. UIDL is another command. You can try them out using telnet to port 110 of the POP server ( telnet:pop.example.com:110 ). After telnet establishes the TCP connection the POP server should send something like "+OK The Microsoft Exchange POP3 service is ready." You can type "CAPA" and hit return, then the POP server should respond with a list of capabilities that it supports (in that state of the session, that is prior to logging in). You can log in by sending "user @name@ and hit return, where @name@ would be changed to your POP account name. You then type "pass @pw@" and hit return, where @pw@ is your password. This sends you password over the network in the clear so someone sniffing the link could easily see your password. Your POP server may require other more secure login. (Don't type in the double quotes in the example above).

Assuming that went well, you can try "CAPA" again now that the session has been established and is in a different state. The list of capabilities may be the same or different depending on the server configuration. At this point you can type "STAT" and hit return. The POP server should return "+OK @x@ @y@" where @x@ is the number of messages and @y@ is the length in bytes of all the messages. Now you can try typing "UIDL" and hit return. the POP server will return a list with "@n@ @uid@" where @n@ is the message number and @uid@ is a unique identifier assigned by the POP server.

Type QUIT and hit return to end the session and close the TCP connection.

Answer 2

The UIDL capability indicates that the optional UIDL command is supported.

POP3 servers may assign a unique number to each incoming mail message. This allows mail to be left on the server after it has been downloaded to the user. Both the mail client and the POP server must support this feature.

Answer 3

It checks if the pop3 server understands (has the CAPAbility) the UIDL command.

The response should be "+OK" or "-ERR" depending on wether the server supports the UIDL command.

The UIDL command returns (if supported) an uniqe identify for each message, so a client can identify messages reliably.

See also: rfc2449(CAPA) and rfc1939(POP3).