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Network Working Group J. Myers Request for Comments: 1939 Carnegie Mellon STD: 53 M. Rose Obsoletes: 1725 Dover Beach Consulting, Inc. Category: Standards Track May 1996 Post Office Protocol - Version 3 Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Table of Contents 1. Introduction ................................................ 2 2. A Short Digression .......................................... 2 3. Basic Operation ............................................. 3 4. The AUTHORIZATION State ..................................... 4 QUIT Command ................................................ 5 5. The TRANSACTION State ....................................... 5 STAT Command ................................................ 6 LIST Command ................................................ 6 RETR Command ................................................ 8 DELE Command ................................................ 8 NOOP Command ................................................ 9 RSET Command ................................................ 9 6. The UPDATE State ............................................ 10 QUIT Command ................................................ 10 7. Optional POP3 Commands ...................................... 11 TOP Command ................................................. 11 UIDL Command ................................................ 12 USER Command ................................................ 13 PASS Command ................................................ 14 APOP Command ................................................ 15 8. Scaling and Operational Considerations ...................... 16 9. POP3 Command Summary ........................................ 18 10. Example POP3 Session ....................................... 19 11. Message Format ............................................. 19 12. References ................................................. 20 13. Security Considerations .................................... 20 14. Acknowledgements ........................................... 20 15. Authors' Addresses ......................................... 21 Appendix A. Differences from RFC 1725 .......................... 22 Myers & Rose Standards Track [Page 1]  RFC 1939 POP3 May 1996 Appendix B. Command Index ...................................... 23 1. Introduction On certain types of smaller nodes in the Internet it is often impractical to maintain a message transport system (MTS). For example, a workstation may not have sufficient resources (cycles, disk space) in order to permit a SMTP server [RFC821] and associated local mail delivery system to be kept resident and continuously running. Similarly, it may be expensive (or impossible) to keep a personal computer interconnected to an IP-style network for long amounts of time (the node is lacking the resource known as "connectivity"). Despite this, it is often very useful to be able to manage mail on these smaller nodes, and they often support a user agent (UA) to aid the tasks of mail handling. To solve this problem, a node which can support an MTS entity offers a maildrop service to these less endowed nodes. The Post Office Protocol - Version 3 (POP3) is intended to permit a workstation to dynamically access a maildrop on a server host in a useful fashion. Usually, this means that the POP3 protocol is used to allow a workstation to retrieve mail that the server is holding for it. POP3 is not intended to provide extensive manipulation operations of mail on the server; normally, mail is downloaded and then deleted. A more advanced (and complex) protocol, IMAP4, is discussed in [RFC1730]. For the remainder of this memo, the term "client host" refers to a host making use of the POP3 service, while the term "server host" refers to a host which offers the POP3 service. 2. A Short Digression This memo does not specify how a client host enters mail into the transport system, although a method consistent with the philosophy of this memo is presented here: When the user agent on a client host wishes to enter a message into the transport system, it establishes an SMTP connection to its relay host and sends all mail to it. This relay host could be, but need not be, the POP3 server host for the client host. Of course, the relay host must accept mail for delivery to arbitrary recipient addresses, that functionality is not required of all SMTP servers. Myers & Rose Standards Track [Page 2]  RFC 1939 POP3 May 1996 3. Basic Operation Initially, the server host starts the POP3 service by listening on TCP port 110. When a client host wishes to make use of the service, it establishes a TCP connection with the server host. When the connection is established, the POP3 server sends a greeting. The client and POP3 server then exchange commands and responses (respectively) until the connection is closed or aborted. Commands in the POP3 consist of a case-insensitive keyword, possibly followed by one or more arguments. All commands are terminated by a CRLF pair. Keywords and arguments consist of printable ASCII characters. Keywords and arguments are each separated by a single SPACE character. Keywords are three or four characters long. Each argument may be up to 40 characters long. Responses in the POP3 consist of a status indicator and a keyword possibly followed by additional information. All responses are terminated by a CRLF pair. Responses may be up to 512 characters long, including the terminating CRLF. There are currently two status indicators: positive ("+OK") and negative ("-ERR"). Servers MUST send the "+OK" and "-ERR" in upper case. Responses to certain commands are multi-line. In these cases, which are clearly indicated below, after sending the first line of the response and a CRLF, any additional lines are sent, each terminated by a CRLF pair. When all lines of the response have been sent, a final line is sent, consisting of a termination octet (decimal code 046, ".") and a CRLF pair. If any line of the multi-line response begins with the termination octet, the line is "byte-stuffed" by pre-pending the termination octet to that line of the response. Hence a multi-line response is terminated with the five octets "CRLF.CRLF". When examining a multi-line response, the client checks to see if the line begins with the termination octet. If so and if octets other than CRLF follow, the first octet of the line (the termination octet) is stripped away. If so and if CRLF immediately follows the termination character, then the response from the POP server is ended and the line containing ".CRLF" is not considered part of the multi-line response. A POP3 session progresses through a number of states during its lifetime. Once the TCP connection has been opened and the POP3 server has sent the greeting, the