The POP3 commands discussed above must be supported by all minimal implementations of POP3 servers.
The optional POP3 commands described below permit a POP3 client greater freedom in message handling, while preserving a simple POP3 server implementation.
NOTE: This memo STRONGLY encourages implementations to support these commands in lieu of developing augmented drop and scan listings. In short, the philosophy of this memo is to put intelligence in the part of the POP3 client and not the POP3 server. TOP msg n Arguments: a message-number (required) which may NOT refer to to a message marked as deleted, and a non-negative number (required) Restrictions: may only be given in the TRANSACTION state Discussion: If the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, the POP3 server sends the headers of the message, the blank line separating the headers from the body, and then the number of lines indicated message's body, being careful to byte-stuff the termination character (as with all multi- line responses). Note that if the number of lines requested by the POP3 client is greater than than the number of lines in the body, then the POP3 server sends the entire message. Possible Responses: +OK top of message follows -ERR no such message Examples: C: TOP 1 10 S: +OK S: <the POP3 server sends the headers of the message, a blank line, and the first 10 lines of the body of the message> S: . ... C: TOP 100 3 S: -ERR no such message UIDL [msg] Arguments: a message-number (optionally) If a message-number is given, it may NOT refer to a message marked as deleted. Restrictions: may only be given in the TRANSACTION state. Discussion: If an argument was given and the POP3 server issues a positive response with a line containing information for that message. This line is called a "unique-id listing" for that message. If no argument was given and the POP3 server issues a positive response, then the response given is multi-line. After the initial +OK, for each message in the maildrop, the POP3 server responds with a line containing information for that message. This line is called a "unique-id listing" for that message. In order to simplify parsing, all POP3 servers are required to use a certain format for unique-id listings. A unique-id listing consists of the message-number of the message, followed by a single space and the unique-id of the message. No information follows the unique-id in the unique-id listing. The unique-id of a message is an arbitrary server-determined string, consisting of characters in the range 0x21 to 0x7E, which uniquely identifies a message within a maildrop and which persists across sessions. The server should never reuse an unique-id in a given maildrop, for as long as the entity using the unique-id exists. Note that messages marked as deleted are not listed. Possible Responses: +OK unique-id listing follows -ERR no such message Examples: C: UIDL S: +OK S: 1 whqtswO00WBw418f9t5JxYwZ S: 2 QhdPYR:00WBw1Ph7x7 S: . ... C: UIDL 2 S: +OK 2 QhdPYR:00WBw1Ph7x7 ... C: UIDL 3 S: -ERR no such message, only 2 messages in maildrop APOP name digest Arguments: a string identifying a mailbox and a MD5 digest string (both required) Restrictions: may only be given in the AUTHORIZATION state after the POP3 greeting Discussion: Normally, each POP3 session starts with a USER/PASS exchange. This results in a server/user-id specific password being sent in the clear on the network. For intermittent use of POP3, this may not introduce a sizable risk. However, many POP3 client implementations connect to the POP3 server on a regular basis -- to check for new mail. Further the interval of session initiation may be on the order of five minutes. Hence, the risk of password capture is greatly enhanced. An alternate method of authentication is required which provides for both origin authentication and replay protection, but which does not involve sending a password in the clear over the network. The APOP command provides this functionality. A POP3 server which implements the APOP command will include a timestamp in its banner greeting. The syntax of the timestamp corresponds to the `msg-id' in [RFC822], and MUST be different each time the POP3 server issues a banner greeting. For example, on a UNIX implementation in which a separate UNIX process is used for each instance of a POP3 server, the syntax of the timestamp might be: <process-ID.clock@hostname> where `process-ID' is the decimal value of the process's PID, clock is the decimal value of the system clock, and hostname is the fully-qualified domain-name corresponding to the host where the POP3 server is running. The POP3 client makes note of this timestamp, and then issues the APOP command. The `name' parameter has identical semantics to the `name' parameter of the USER command. The `digest' parameter is calculated by applying the MD5 algorithm [RFC1321] to a string consisting of the timestamp (including angle-brackets) followed by a shared secret. This shared secret is a string known only to the POP3 client and server. Great care should be taken to prevent unauthorized disclosure of the secret, as knowledge of the secret will allow any entity to successfully masquerade as the named user. The `digest' parameter itself is a 16-octet value which is sent in hexadecimal format, using lower-case ASCII characters. When the POP3 server receives the APOP command, it verifies the digest provided. If the digest is correct, the POP3 server issues a positive response, and the POP3 session enters the TRANSACTION state. Otherwise, a negative response is issued and the POP3 session remains in the AUTHORIZATION state. Note that as the length of the shared secret increases, so does the difficulty of deriving it. As such, shared secrets should be long strings (considerably longer than the 8-character example shown below). Possible Responses: +OK maildrop locked and ready -ERR permission denied Examples: S: +OK POP3 server ready <firstname.lastname@example.org> C: APOP mrose c4c9334bac560ecc979e58001b3e22fb S: +OK maildrop has 1 message (369 octets) In this example, the shared secret is the string `tan- staaf'. Hence, the MD5 algorithm is applied to the string <email@example.com>tanstaaf which produces a digest value of c4c9334bac560ecc979e58001b3e22fb