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

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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.

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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 session enters the AUTHORIZATION

state. In this state, the client must identify itself to the POP3

server. Once the client has successfully done this, the server

acquires resources associated with the client's maildrop, and the

session enters the TRANSACTION state. In this state, the client

requests actions on the part of the POP3 server. When the client has

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issued the QUIT command, the session enters the UPDATE state. In

this state, the POP3 server releases any resources acquired during

the TRANSACTION state and says goodbye. The TCP connection is then

closed.

A server MUST respond to an unrecognized, unimplemented, or

syntactically invalid command by responding with a negative status

indicator. A server MUST respond to a command issued when the

session is in an incorrect state by responding with a negative status

indicator. There is no general method for a client to distinguish

between a server which does not implement an optional command and a

server which is unwilling or unable to process the command.

A POP3 server MAY have an inactivity autologout timer. Such a timer

MUST be of at least 10 minutes' duration. The receipt of any command

from the client during that interval should suffice to reset the

autologout timer. When the timer expires, the session does NOT enter

the UPDATE state--the server should close the TCP connection without

removing any messages or sending any response to the client.

4. The AUTHORIZATION State

Once the TCP connection has been opened by a POP3 client, the POP3

server issues a one line greeting. This can be any positive

response. An example might be:

S: +OK POP3 server ready

The POP3 session is now in the AUTHORIZATION state. The client must

now identify and authenticate itself to the POP3 server. Two

possible mechanisms for doing this are described in this document,

the USER and PASS command combination and the APOP command. Both

mechanisms are described later in this document. Additional

authentication mechanisms are described in [RFC1734]. While there is

no single authentication mechanism that is required of all POP3

servers, a POP3 server must of course support at least one

authentication mechanism.

Once the POP3 server has determined through the use of any

authentication command that the client should be given access to the

appropriate maildrop, the POP3 server then acquires an exclusive-

access lock on the maildrop, as necessary to prevent messages from

being modified or removed before the session enters the UPDATE state.

If the lock is successfully acquired, the POP3 server responds with a

positive status indicator. The POP3 session now enters the

TRANSACTION state, with no messages marked as deleted. If the

maildrop cannot be opened for some reason (for example, a lock can

not be acquired, the client is denied access to the appropriate

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maildrop, or the maildrop cannot be parsed), the POP3 server responds

with a negative status indicator. (If a lock was acquired but the

POP3 server intends to respond with a negative status indicator, the

POP3 server must release the lock prior to rejecting the command.)

After returning a negative status indicator, the server may close the

connection. If the server does not close the connection, the client

may either issue a new authentication command and start again, or the

client may issue the QUIT command.

After the POP3 server has opened the maildrop, it assigns a message-

number to each message, and notes the size of each message in octets.

The first message in the maildrop is assigned a message-number of

"1", the second is assigned "2", and so on, so that the nth message

in a maildrop is assigned a message-number of "n". In POP3 commands

and responses, all message-numbers and message sizes are expressed in

base-10 (i.e., decimal).

Here is the summary for the QUIT command when used in the

AUTHORIZATION state:

QUIT

Arguments: none

Restrictions: none

Possible Responses:

+OK

Examples:

C: QUIT

S: +OK dewey POP3 server signing off

5. The TRANSACTION State

Once the client has successfully identified itself to the POP3 server

and the POP3 server has locked and opened the appropriate maildrop,

the POP3 session is now in the TRANSACTION state. The client may now

issue any of the following POP3 commands repeatedly. After each

command, the POP3 server issues a response. Eventually, the client

issues the QUIT command and the POP3 session enters the UPDATE state.

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Here are the POP3 commands valid in the TRANSACTION state:

STAT

Arguments: none

Restrictions:

may only be given in the TRANSACTION state

Discussion:

The POP3 server issues a positive response with a line

containing information for the maildrop. This line is

called a "drop listing" for that maildrop.

In order to simplify parsing, all POP3 servers are

required to use a certain format for drop listings. The

positive response consists of "+OK" followed by a single

space, the number of messages in the maildrop, a single

space, and the size of the maildrop in octets. This memo

makes no requirement on what follows the maildrop size.

Minimal implementations should just end that line of the

response with a CRLF pair. More advanced implementations

may include other information.

NOTE: This memo STRONGLY discourages implementations

from supplying additional information in the drop

listing. Other, optional, facilities are discussed

later on which permit the client to parse the messages

in the maildrop.

Note that messages marked as deleted are not counted in

either total.

Possible Responses:

+OK nn mm

Examples:

C: STAT

S: +OK 2 320

LIST [msg]

Arguments:

a message-number (optional), which, if present, may NOT

refer to a message marked as deleted

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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 "scan 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 also called a

"scan listing" for that message. If there are no

messages in the maildrop, then the POP3 server responds

with no scan listings--it issues a positive response

followed by a line containing a termination octet and a

CRLF pair.

In order to simplify parsing, all POP3 servers are

required to use a certain format for scan listings. A

scan listing consists of the message-number of the

message, followed by a single space and the exact size of

the message in octets. Methods for calculating the exact

size of the message are described in the "Message Format"

section below. This memo makes no requirement on what

follows the message size in the scan listing. Minimal

implementations should just end that line of the response

with a CRLF pair. More advanced implementations may

include other information, as parsed from the message.

NOTE: This memo STRONGLY discourages implementations

from supplying additional information in the scan

listing. Other, optional, facilities are discussed

later on which permit the client to parse the messages

in the maildrop.

Note that messages marked as deleted are not listed.

Possible Responses:

+OK scan listing follows

-ERR no such message

Examples:

C: LIST

S: +OK 2 messages (320 octets)

S: 1 120

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S: 2 200

S: .

...

C: LIST 2

S: +OK 2 200

...

C: LIST 3

S: -ERR no such message, only 2 messages in maildrop

RETR msg

Arguments:

a message-number (required) which may NOT refer to a

message marked as deleted

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 message corresponding to the given

message-number, being careful to byte-stuff the termination

character (as with all multi-line responses).

Possible Responses:

+OK message follows

-ERR no such message

Examples:

C: RETR 1

S: +OK 120 octets

S: <the POP3 server sends the entire message here>

S: .

DELE msg

Arguments:

a message-number (required) which may NOT refer to a

message marked as deleted

Restrictions:

may only be given in the TRANSACTION state

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Discussion:

The POP3 server marks the message as deleted. Any future

reference to the message-number associated with the message

in a POP3 command generates an error. The POP3 server does

not actually delete the message until the POP3 session

enters the UPDATE state.

Possible Responses:

+OK message deleted

-ERR no such message

Examples:

C: DELE 1

S: +OK message 1 deleted

...

C: DELE 2

S: -ERR message 2 already deleted

NOOP

Arguments: none

Restrictions:

may only be given in the TRANSACTION state

Discussion:

The POP3 server does nothing, it merely replies with a

positive response.

Possible Responses:

+OK

Examples:

C: NOOP

S: +OK

RSET

Arguments: none

Restrictions:

may only be given in the TRANSACTION state

Discussion:

If any messages have been marked as deleted by the POP3

server, they are unmarked. The POP3 server then replies

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with a positive response.

Possible Responses:

+OK

Examples:

C: RSET

S: +OK maildrop has 2 messages (320 octets)

6. The UPDATE State

When the client issues the QUIT command from the TRANSACTION state,

the POP3 session enters the UPDATE state. (Note that if the client

issues the QUIT command from the AUTHORIZATION state, the POP3

session terminates but does NOT enter the UPDATE state.)

If a session terminates for some reason other than a client-issued

QUIT command, the POP3 session does NOT enter the UPDATE state and

MUST not remove any messages from the maildrop.

QUIT

Arguments: none

Restrictions: none

Discussion:

The POP3 server removes all messages marked as deleted

from the maildrop and replies as to the status of this

operation. If there is an error, such as a resource

shortage, encountered while removing messages, the

maildrop may result in having some or none of the messages

marked as deleted be removed. In no case may the server

remove any messages not marked as deleted.

Whether the removal was successful or not, the server

then releases any exclusive-access lock on the maildrop

and closes the TCP connection.

Possible Responses:

+OK

-ERR some deleted messages not removed

Examples:

C: QUIT

S: +OK dewey POP3 server signing off (maildrop empty)

...

C: QUIT

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S: +OK dewey POP3 server signing off (2 messages left)

...

7. Optional POP3 Commands

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

of lines (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 of the 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

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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 (optional), which, if present, 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 one to 70 characters in the range 0x21

to 0x7E, which uniquely identifies a message within a

maildrop and which persists across sessions. This

persistence is required even if a session ends without

entering the UPDATE state. 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.

While it is generally preferable for server implementations

to store arbitrarily assigned unique-ids in the maildrop,

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this specification is intended to permit unique-ids to be

calculated as a hash of the message. Clients should be able

to handle a situation where two identical copies of a

message in a maildrop have the same unique-id.

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

USER name

Arguments:

a string identifying a mailbox (required), which is of

significance ONLY to the server

Restrictions:

may only be given in the AUTHORIZATION state after the POP3

greeting or after an unsuccessful USER or PASS command

Discussion:

To authenticate using the USER and PASS command

combination, the client must first issue the USER

command. If the POP3 server responds with a positive

status indicator ("+OK"), then the client may issue

either the PASS command to complete the authentication,

or the QUIT command to terminate the POP3 session. If

the POP3 server responds with a negative status indicator

("-ERR") to the USER command, then the client may either

issue a new authentication command or may issue the QUIT

command.

The server may return a positive response even though no

such mailbox exists. The server may return a negative

response if mailbox exists, but does not permit plaintext

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password authentication.

Possible Responses:

+OK name is a valid mailbox

-ERR never heard of mailbox name

Examples:

C: USER frated

S: -ERR sorry, no mailbox for frated here

...

C: USER mrose

S: +OK mrose is a real hoopy frood

PASS string

Arguments:

a server/mailbox-specific password (required)

Restrictions:

may only be given in the AUTHORIZATION state immediately

after a successful USER command

Discussion:

When the client issues the PASS command, the POP3 server

uses the argument pair from the USER and PASS commands to

determine if the client should be given access to the

appropriate maildrop.

Since the PASS command has exactly one argument, a POP3

server may treat spaces in the argument as part of the

password, instead of as argument separators.

Possible Responses:

+OK maildrop locked and ready

-ERR invalid password

-ERR unable to lock maildrop

Examples:

C: USER mrose

S: +OK mrose is a real hoopy frood

C: PASS secret

S: -ERR maildrop already locked

...

C: USER mrose

S: +OK mrose is a real hoopy frood

C: PASS secret

S: +OK mrose's maildrop has 2 messages (320 octets)

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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 or after an unsuccessful USER or PASS command

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

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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 <1896.697170952@dbc.mtview.ca.us>

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

<1896.697170952@dbc.mtview.ca.us>tanstaaf

which produces a digest value of

c4c9334bac560ecc979e58001b3e22fb

8. Scaling and Operational Considerations

Since some of the optional features described above were added to the

POP3 protocol, experience has accumulated in using them in large-

scale commercial post office operations where most of the users are

unrelated to each other. In these situations and others, users and

vendors of POP3 clients have discovered that the combination of using

the UIDL command and not issuing the DELE command can provide a weak

version of the "maildrop as semi-permanent repository" functionality

normally associated with IMAP. Of course the other capabilities of

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IMAP, such as polling an existing connection for newly arrived

messages and supporting multiple folders on the server, are not

present in POP3.

When these facilities are used in this way by casual users, there has

been a tendency for already-read messages to accumulate on the server

without bound. This is clearly an undesirable behavior pattern from

the standpoint of the server operator. This situation is aggravated

by the fact that the limited capabilities of the POP3 do not permit

efficient handling of maildrops which have hundreds or thousands of

messages.

Consequently, it is recommended that operators of large-scale multi-

user servers, especially ones in which the user's only access to the

maildrop is via POP3, consider such options as:

* Imposing a per-user maildrop storage quota or the like.

A disadvantage to this option is that accumulation of messages may

result in the user's inability to receive new ones into the

maildrop. Sites which choose this option should be sure to inform

users of impending or current exhaustion of quota, perhaps by

inserting an appropriate message into the user's maildrop.

* Enforce a site policy regarding mail retention on the server.

Sites are free to establish local policy regarding the storage and

retention of messages on the server, both read and unread. For

example, a site might delete unread messages from the server after

60 days and delete read messages after 7 days. Such message

deletions are outside the scope of the POP3 protocol and are not

considered a protocol violation.

Server operators enforcing message deletion policies should take

care to make all users aware of the policies in force.

Clients must not assume that a site policy will automate message

deletions, and should continue to explicitly delete messages using

the DELE command when appropriate.

It should be noted that enforcing site message deletion policies

may be confusing to the user community, since their POP3 client

may contain configuration options to leave mail on the server

which will not in fact be supported by the server.

One special case of a site policy is that messages may only be

downloaded once from the server, and are deleted after this has

been accomplished. This could be implemented in POP3 server

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software by the following mechanism: "following a POP3 login by a

client which was ended by a QUIT, delete all messages downloaded

during the session with the RETR command". It is important not to

delete messages in the event of abnormal connection termination

(ie, if no QUIT was received from the client) because the client

may not have successfully received or stored the messages.

Servers implementing a download-and-delete policy may also wish to

disable or limit the optional TOP command, since it could be used

as an alternate mechanism to download entire messages.

9. POP3 Command Summary

Minimal POP3 Commands:

USER name valid in the AUTHORIZATION state

PASS string

QUIT

STAT valid in the TRANSACTION state

LIST [msg]

RETR msg

DELE msg

NOOP

RSET

QUIT

Optional POP3 Commands:

APOP name digest valid in the AUTHORIZATION state

TOP msg n valid in the TRANSACTION state

UIDL [msg]

POP3 Replies:

+OK

-ERR

Note that with the exception of the STAT, LIST, and UIDL commands,

the reply given by the POP3 server to any command is significant

only to "+OK" and "-ERR". Any text occurring after this reply

may be ignored by the client.