IMAP Response Codes

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.

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Abstract

IMAP responses consist of a response type (OK, NO, BAD), an optional machine-readable response code, and a human-readable text.

This document collects and documents a variety of machine-readable response codes, for better interoperation and error reporting.

1. Introduction

Section 7.1 of [RFC3501] defines a number of response codes that can help tell an IMAP client why a command failed. However, experience has shown that more codes are useful. For example, it is useful for a client to know that an authentication attempt failed because of a server problem as opposed to a password problem.

Currently, many IMAP servers use English-language, human-readable text to describe these errors, and a few IMAP clients attempt to translate this text into the user's language.

This document names a variety of errors as response codes. It is based on errors that have been checked and reported on in some IMAP server implementations, and on the needs of some IMAP clients.

This document doesn't require any servers to test for these errors or any clients to test for these names. It only names errors for better reporting and handling.

2. Conventions Used in This Document

Formal syntax is defined by [RFC5234] as modified by [RFC3501].

Example lines prefaced by "C:" are sent by the client and ones prefaced by "S:" by the server. "[...]" means elision.

3. Response Codes

This section defines all the new response codes. Each definition is followed by one or more examples.

UNAVAILABLE

Temporary failure because a subsystem is down. For example, an IMAP server that uses a Lightweight Directory Access Protocol (LDAP) or Radius server for authentication might use this response code when the LDAP/Radius server is down.

C: a LOGIN "fred" "foo"
S: a NO [UNAVAILABLE] User's backend down for maintenance

AUTHENTICATIONFAILED

Authentication failed for some reason on which the server is unwilling to elaborate. Typically, this includes "unknown user" and "bad password".

This is the same as not sending any response code, except that when a client sees AUTHENTICATIONFAILED, it knows that the problem wasn't, e.g., UNAVAILABLE, so there's no point in trying the same login/password again later.

C: b LOGIN "fred" "foo"
S: b NO [AUTHENTICATIONFAILED] Authentication failed

AUTHORIZATIONFAILED

Authentication succeeded in using the authentication identity, but the server cannot or will not allow the authentication identity to act as the requested authorization identity. This is only applicable when the authentication and authorization identities are different.

         C: c1 AUTHENTICATE PLAIN
         [...]
         S: c1 NO [AUTHORIZATIONFAILED] No such authorization-ID
         
         C: c2 AUTHENTICATE PLAIN
         [...]
         S: c2 NO [AUTHORIZATIONFAILED] Authenticator is not an admin

EXPIRED

Either authentication succeeded or the server no longer had the necessary data; either way, access is no longer permitted using that passphrase. The client or user should get a new passphrase.

C: d login "fred" "foo"
S: d NO [EXPIRED] That password isn't valid any more

PRIVACYREQUIRED

The operation is not permitted due to a lack of privacy. If Transport Layer Security (TLS) is not in use, the client could try STARTTLS (see Section 6.2.1 of [RFC3501]) and then repeat the operation.

C: d login "fred" "foo"
S: d NO [PRIVACYREQUIRED] Connection offers no privacy

C: d select inbox
S: d NO [PRIVACYREQUIRED] Connection offers no privacy

CONTACTADMIN

The user should contact the system administrator or support desk.

         C: e login "fred" "foo"
         S: e OK [CONTACTADMIN]

NOPERM

The access control system (e.g., Access Control List (ACL), see [RFC4314]) does not permit this user to carry out an operation, such as selecting or creating a mailbox.

C: f select "/archive/projects/experiment-iv"
S: f NO [NOPERM] Access denied

INUSE

An operation has not been carried out because it involves sawing off a branch someone else is sitting on. Someone else may be holding an exclusive lock needed for this operation, or the operation may involve deleting a resource someone else is using, typically a mailbox.

The operation may succeed if the client tries again later.

C: g delete "/archive/projects/experiment-iv"
S: g NO [INUSE] Mailbox in use

EXPUNGEISSUED

Someone else has issued an EXPUNGE for the same mailbox. The client may want to issue NOOP soon. [RFC2180] discusses this subject in depth.

C: h search from fred@example.com
S: * SEARCH 1 2 3 5 8 13 21 42
S: h OK [EXPUNGEISSUED] Search completed

CORRUPTION

The server discovered that some relevant data (e.g., the mailbox) are corrupt. This response code does not include any information about what's corrupt, but the server can write that to its logfiles.

C: i select "/archive/projects/experiment-iv"
S: i NO [CORRUPTION] Cannot open mailbox

SERVERBUG

The server encountered a bug in itself or violated one of its own invariants.

C: j select "/archive/projects/experiment-iv"
S: j NO [SERVERBUG] This should not happen

CLIENTBUG

The server has detected a client bug. This can accompany all of OK, NO, and BAD, depending on what the client bug is.

         C: k1 select "/archive/projects/experiment-iv"
         [...]
         S: k1 OK [READ-ONLY] Done
         C: k2 status "/archive/projects/experiment-iv" (messages)
         [...]
         S: k2 OK [CLIENTBUG] Done

CANNOT

The operation violates some invariant of the server and can never succeed.

C: l create "///////"
S: l NO [CANNOT] Adjacent slashes are not supported

LIMIT

The operation ran up against an implementation limit of some kind, such as the number of flags on a single message or the number of flags used in a mailbox.

C: m STORE 42 FLAGS f1 f2 f3 f4 f5 ... f250
S: m NO [LIMIT] At most 32 flags in one mailbox supported

OVERQUOTA

The user would be over quota after the operation. (The user may or may not be over quota already.)

Note that if the server sends OVERQUOTA but doesn't support the

IMAP QUOTA extension defined by [RFC2087], then there is a

quota, but the client cannot find out what the quota is.

         C: n1 uid copy 1:* oldmail
         S: n1 NO [OVERQUOTA] Sorry

C: n2 uid copy 1:* oldmail
S: n2 OK [OVERQUOTA] You are now over your soft quota

ALREADYEXISTS

The operation attempts to create something that already exists, such as when the CREATE or RENAME directories attempt to create a mailbox and there is already one of that name.

C: o RENAME this that
S: o NO [ALREADYEXISTS] Mailbox "that" already exists

NONEXISTENT

The operation attempts to delete something that does not exist. Similar to ALREADYEXISTS.

C: p RENAME this that
S: p NO [NONEXISTENT] No such mailbox

4. Formal Syntax

The following syntax specification uses the Augmented Backus-Naur Form (ABNF) notation as specified in [RFC5234]. [RFC3501] defines the non-terminal "resp-text-code".

Except as noted otherwise, all alphabetic characters are case- insensitive. The use of upper or lowercase characters to define token strings is for editorial clarity only.

        resp-text-code =/ "UNAVAILABLE" / "AUTHENTICATIONFAILED" /
                         "AUTHORIZATIONFAILED" / "EXPIRED" /
                         "PRIVACYREQUIRED" / "CONTACTADMIN" / "NOPERM" /
                         "INUSE" / "EXPUNGEISSUED" / "CORRUPTION" /
                         "SERVERBUG" / "CLIENTBUG" / "CANNOT" /
                         "LIMIT" / "OVERQUOTA" / "ALREADYEXISTS" /
                         "NONEXISTENT"

5. Security Considerations

Revealing information about a passphrase to unauthenticated IMAP clients causes bad karma.

Response codes are easier to parse than human-readable text. This can amplify the consequences of an information leak. For example, selecting a mailbox can fail because the mailbox doesn't exist, because the user doesn't have the "l" right (right to know the mailbox exists) or "r" right (right to read the mailbox). If the server sent different responses in the first two cases in the past, only malevolent clients would discover it. With response codes it's possible, perhaps probable, that benevolent clients will forward the leaked information to the user. Server authors are encouraged to be particularly careful with the NOPERM and authentication-related responses.

6. IANA Considerations

The IANA has created the IMAP Response Codes registry. The registry has been populated with the following codes:

      NEWNAME              RFC 2060 (obsolete)
      REFERRAL             RFC 2221
      ALERT                RFC 3501
      BADCHARSET           RFC 3501
      PARSE                RFC 3501
      PERMANENTFLAGS       RFC 3501
      READ-ONLY            RFC 3501
      READ-WRITE           RFC 3501
      TRYCREATE            RFC 3501
      UIDNEXT              RFC 3501
      UIDVALIDITY          RFC 3501
      UNSEEN               RFC 3501
      UNKNOWN-CTE          RFC 3516
      UIDNOTSTICKY         RFC 4315
      APPENDUID            RFC 4315
      COPYUID              RFC 4315
      URLMECH              RFC 4467
      TOOBIG               RFC 4469
      BADURL               RFC 4469
      HIGHESTMODSEQ        RFC 4551
      NOMODSEQ             RFC 4551
      MODIFIED             RFC 4551
      COMPRESSIONACTIVE    RFC 4978
      CLOSED               RFC 5162
      NOTSAVED             RFC 5182
      BADCOMPARATOR        RFC 5255
      ANNOTATE             RFC 5257
      ANNOTATIONS          RFC 5257
      TEMPFAIL             RFC 5259
      MAXCONVERTMESSAGES   RFC 5259
      MAXCONVERTPARTS      RFC 5259
      NOUPDATE             RFC 5267
      METADATA             RFC 5464
      NOTIFICATIONOVERFLOW RFC 5465
      BADEVENT             RFC 5465
      UNDEFINED-FILTER     RFC 5466
      UNAVAILABLE          RFC 5530
      AUTHENTICATIONFAILED RFC 5530
      AUTHORIZATIONFAILED  RFC 5530
      
      EXPIRED              RFC 5530
      PRIVACYREQUIRED      RFC 5530
      CONTACTADMIN         RFC 5530
      NOPERM               RFC 5530
      INUSE                RFC 5530
      EXPUNGEISSUED        RFC 5530
      CORRUPTION           RFC 5530
      SERVERBUG            RFC 5530
      CLIENTBUG            RFC 5530
      CANNOT               RFC 5530
      LIMIT                RFC 5530
      OVERQUOTA            RFC 5530
      ALREADYEXISTS        RFC 5530
      NONEXISTENT          RFC 5530

The new registry can be extended by sending a registration request to IANA. IANA will forward this request to a Designated Expert, appointed by the responsible IESG Area Director, CCing it to the IMAP Extensions mailing list at <ietf-imapext@imc.org> (or a successor designated by the Area Director). After either allowing 30 days for community input on the IMAP Extensions mailing list or a successful IETF Last Call, the expert will determine the appropriateness of the registration request and either approve or disapprove the request by sending a notice of the decision to the requestor, CCing the IMAP Extensions mailing list and IANA. A denial notice must be justified by an explanation, and, in cases where it is possible, concrete suggestions on how the request can be modified so as to become acceptable should be provided.

For each response code, the registry contains a list of relevant RFCs that describe (or extend) the response code and an optional response code status description, such as "obsolete" or "reserved to prevent collision with deployed software". (Note that in the latter case, the RFC number can be missing.) Presence of the response code status description means that the corresponding response code is NOT RECOMMENDED for widespread use.

The intention is that any future allocation will be accompanied by a published RFC (including direct submissions to the RFC Editor). But in order to allow for the allocation of values prior to the RFC being approved for publication, the Designated Expert can approve allocations once it seems clear that an RFC will be published, for example, before requesting IETF LC for the document.

The Designated Expert can also approve registrations for response codes used in deployed software when no RFC exists. Such registrations must be marked as "reserved to prevent collision with deployed software".

Response code registrations may not be deleted; response codes that are no longer believed appropriate for use (for example, if there is a problem with the syntax of said response code or if the specification describing it was moved to Historic) should be marked "obsolete" in the registry, clearly marking the lists published by IANA.

7. Acknowledgements

Peter Coates, Mark Crispin, Philip Guenther, Alexey Melnikov, Ken Murchison, Chris Newman, Timo Sirainen, Philip Van Hoof, Dale Wiggins, and Sarah Wilkin helped with this document.

8. References

8.1. Normative References

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.
   
   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234, January
              2008.

9. Informative References

   [RFC2087]  Myers, J., "IMAP4 QUOTA extension", RFC 2087, January
              1997.
   
   [RFC2180]  Gahrns, M., "IMAP4 Multi-Accessed Mailbox Practice", RFC
              2180, July 1997.
   
   [RFC4314]  Melnikov, A., "IMAP4 Access Control List (ACL) Extension",
              RFC 4314, December 2005.

Author's Address

Arnt Gulbrandsen
Oryx Mail Systems GmbH
Schweppermannstr. 8
D-81671 Muenchen
Germany

   Fax: +49 89 4502 9758
   EMail: arnt@oryx.com