Network Working Group R. Siemborski, Ed. Request for Comments: 4954 Google, Inc. Obsoletes: 2554 A. Melnikov, Ed. Updates: 3463 Isode Limited Category: Standards Track July 2007
SMTP Service Extension for Authentication
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.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document defines a Simple Mail Transport Protocol (SMTP) extension whereby an SMTP client may indicate an authentication mechanism to the server, perform an authentication protocol exchange, and optionally negotiate a security layer for subsequent protocol interactions during this session. This extension includes a profile of the Simple Authentication and Security Layer (SASL) for SMTP.
This document defines a Simple Mail Transport Protocol (SMTP) extension whereby an SMTP client may indicate an authentication mechanism to the server, perform an authentication protocol exchange, optionally negotiate a security layer for subsequent protocol interactions during this session and, during a mail transaction, optionally specify a mailbox associated with the identity that submitted the message to the mail delivery system.
This extension includes a profile of the Simple Authentication and Security Layer (SASL) for SMTP.
When compared to RFC 2554, this document deprecates use of the 538 response code, adds a new Enhanced Status Code, adds a requirement to support SASLprep profile for preparing authorization identities, recommends use of RFC 3848 transmission types in the Received trace header field, and clarifies interaction with SMTP PIPELINING [PIPELINING] extension.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEYWORDS].
In examples, "C:" and "S:" indicate lines sent by the client and server, respectively.
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1. The name of this [SMTP] service extension is "Authentication".
2. The EHLO keyword value associated with this extension is "AUTH".
3. The AUTH EHLO keyword contains as a parameter a space-separated list of the names of available [SASL] mechanisms. The list of available mechanisms MAY change after a successful STARTTLS command [SMTP-TLS].
5. An optional parameter using the keyword "AUTH" is added to the MAIL FROM command, and extends the maximum line length of the MAIL FROM command by 500 characters.
6. This extension is appropriate for the submission protocol [SUBMIT].
Arguments: mechanism: A string identifying a [SASL] authentication mechanism.
initial-response: An optional initial client response. If present, this response MUST be encoded as described in Section 4 of [BASE64] or contain a single character "=".
Restrictions: After an AUTH command has been successfully completed, no more AUTH commands may be issued in the same session. After a successful AUTH command completes, a server MUST reject any further AUTH commands with a 503 reply.
The AUTH command is not permitted during a mail transaction. An AUTH command issued during a mail transaction MUST be rejected with a 503 reply.
Discussion: The AUTH command initiates a [SASL] authentication exchange between the client and the server. The client identifies the SASL mechanism to use with the first parameter of the AUTH command. If the server supports the requested authentication mechanism, it performs the SASL exchange to authenticate the
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user. Optionally, it also negotiates a security layer for subsequent protocol interactions during this session. If the requested authentication mechanism is invalid (e.g., is not supported or requires an encryption layer), the server rejects the AUTH command with a 504 reply. If the server supports the [ESMTP-CODES] extension, it SHOULD return a 5.5.4 enhanced response code.
The SASL authentication exchange consists of a series of server challenges and client responses that are specific to the chosen [SASL] mechanism.
A server challenge is sent as a 334 reply with the text part containing the [BASE64] encoded string supplied by the SASL mechanism. This challenge MUST NOT contain any text other than the BASE64 encoded challenge.
A client response consists of a line containing a [BASE64] encoded string. If the client wishes to cancel the authentication exchange, it issues a line with a single "*". If the server receives such a response, it MUST reject the AUTH command by sending a 501 reply.
The optional initial response argument to the AUTH command is used to save a round-trip when using authentication mechanisms that support an initial client response. If the initial response argument is omitted and the chosen mechanism requires an initial client response, the server MUST proceed as defined in Section 5.1 of [SASL]. In SMTP, a server challenge that contains no data is defined as a 334 reply with no text part. Note that there is still a space following the reply code, so the complete response line is "334 ".
Note that the AUTH command is still subject to the line length limitations defined in [SMTP]. If use of the initial response argument would cause the AUTH command to exceed this length, the client MUST NOT use the initial response parameter (and instead proceed as defined in Section 5.1 of [SASL]).
If the client is transmitting an initial response of zero length, it MUST instead transmit the response as a single equals sign ("="). This indicates that the response is present, but contains no data.
If the client uses an initial-response argument to the AUTH command with a SASL mechanism in which the client does not begin the authentication exchange, the server MUST reject the
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AUTH command with a 501 reply. Servers using the enhanced status codes extension [ESMTP-CODES] SHOULD return an enhanced status code of 5.7.0 in this case.
If the server cannot [BASE64] decode any client response, it MUST reject the AUTH command with a 501 reply (and an enhanced status code of 5.5.2). If the client cannot BASE64 decode any of the server's challenges, it MUST cancel the authentication using the "*" response. In particular, servers and clients MUST reject (and not ignore) any character not explicitly allowed by the BASE64 alphabet, and MUST reject any sequence of BASE64 characters that contains the pad character ('=') anywhere other than the end of the string (e.g., "=AAA" and "AAA=BBB" are not allowed).
Note that these [BASE64] strings can be much longer than normal SMTP commands. Clients and servers MUST be able to handle the maximum encoded size of challenges and responses generated by their supported authentication mechanisms. This requirement is independent of any line length limitations the client or server may have in other parts of its protocol implementation. (At the time of writing of this document, 12288 octets is considered to be a sufficient line length limit for handling of deployed authentication mechanisms.) If, during an authentication exchange, the server receives a line that is longer than the server's authentication buffer, the server fails the AUTH command with the 500 reply. Servers using the enhanced status codes extension [ESMTP-CODES] SHOULD return an enhanced status code of 5.5.6 in this case.
The authorization identity generated by this [SASL] exchange is a "simple username" (in the sense defined in [SASLprep]), and both client and server SHOULD (*) use the [SASLprep] profile of the [StringPrep] algorithm to prepare these names for transmission or comparison. If preparation of the authorization identity fails or results in an empty string (unless it was transmitted as the empty string), the server MUST fail the authentication.
(*) Note: Future revision of this specification may change this requirement to MUST. Currently, the SHOULD is used in order to avoid breaking the majority of existing implementations.
If the server is unable to authenticate the client, it SHOULD reject the AUTH command with a 535 reply unless a more specific error code is appropriate. Should the client successfully complete the exchange, the SMTP server issues a 235 reply. (Note that the SMTP protocol doesn't support the SASL feature of returning additional
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data with a successful outcome.) These status codes, along with others defined by this extension, are discussed in Section 6 of this document.
If a security layer is negotiated during the SASL exchange, it takes effect for the client on the octet immediately following the CRLF that concludes the last response generated by the client. For the server, it takes effect immediately following the CRLF of its success reply.
When a security layer takes effect, the SMTP protocol is reset to the initial state (the state in SMTP after a server issues a 220 service ready greeting). The server MUST discard any knowledge obtained from the client, such as the EHLO argument, which was not obtained from the SASL negotiation itself. Likewise, the client MUST discard any knowledge obtained from the server, such as the list of SMTP service extensions, which was not obtained from the SASL negotiation itself. (Note that a client MAY compare the advertised SASL mechanisms before and after authentication in order to detect an active down- negotiation attack).
The client SHOULD send an EHLO command as the first command after a successful SASL negotiation that results in the enabling of a security layer.
When an entity (whether it is the client or the server end) is sending data, and both [TLS] and SASL security layers are in effect, the TLS encoding MUST be applied after the SASL encoding, regardless of the order in which the layers were negotiated.
The service name specified by this protocol's profile of SASL is "smtp". This service name is also to be used for the [SUBMIT] protocol.
If an AUTH command fails, the client MAY proceed without authentication. Alternatively, the client MAY try another authentication mechanism or present different credentials by issuing another AUTH
Note: A server implementation MUST implement a configuration in which it does NOT permit any plaintext password mechanisms, unless either the STARTTLS [SMTP-TLS] command has been negotiated or some other mechanism that protects the session from password snooping has been provided. Server sites SHOULD NOT use any configuration which permits a plaintext password mechanism without such a protection mechanism against password snooping.
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To ensure interoperability, client and server implementations of this extension MUST implement the [PLAIN] SASL mechanism running over TLS [TLS] [SMTP-TLS]. See also Section 15 for additional requirements on implementations of [PLAIN] over [TLS].
Note that many existing client and server implementations implement CRAM-MD5 [CRAM-MD5] SASL mechanism. In order to ensure interoperability with deployed software, new implementations MAY implement it; however, implementations should be aware that this SASL mechanism doesn't provide any server authentication. Note that at the time of writing of this document the SASL Working Group is working on several replacement SASL mechanisms that provide server authentication and other features.
When the AUTH command is used together with the [PIPELINING] extension, it MUST be the last command in a pipelined group of commands. The only exception to this rule is when the AUTH command contains an initial response for a SASL mechanism that allows the client to send data first, the SASL mechanism is known to complete in one round-trip, and a security layer is not negotiated by the client. Two examples of such SASL mechanisms are PLAIN [PLAIN] and EXTERNAL [SASL].
Here is another client that is attempting AUTH PLAIN under a TLS layer, this time without the initial response. Parts of the negotiation before the TLS layer was established have been omitted:
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... TLS negotiation proceeds, further commands protected by TLS layer ... C: EHLO client.example.com S: 250-smtp.example.com Hello client.example.com S: 250 AUTH GSSAPI DIGEST-MD5 PLAIN C: AUTH PLAIN (note: there is a single space following the 334 on the following line) S: 334 C: dGVzdAB0ZXN0ADEyMzQ= S: 235 2.7.0 Authentication successful
Here is an example using CRAM-MD5 [CRAM-MD5], a mechanism in which the client does not begin the authentication exchange, and includes a server challenge:
Here is an example of a client attempting AUTH EXTERNAL under TLS, using the derived authorization ID (and thus a zero-length initial client response).
Arguments: A <mailbox> (see Section 4.1.2 of [SMTP]) that is associated with the identity that submitted the message to the delivery system, or the two character sequence "<>" indicating such an identity is unknown or insufficiently authenticated. To comply with restrictions imposed on ESMTP parameters, the <mailbox> is encoded inside an xtext. The syntax of an xtext is described in Section 4 of [ESMTP-DSN].
Note: For the purposes of this discussion, "authenticated identity" refers to the identity (if any) derived from the authorization identity of previous AUTH command, while the terms "authorized identity" and "supplied <mailbox>" refer to the sender identity that is being associated with a particular message. Note that one authenticated identity may be able to identify messages as being sent by any number of authorized identities within a single session. For example, this may be the case when an SMTP server (one authenticated identity) is processing its queue (many messages with distinct authorized identities).
Discussion: The optional AUTH parameter to the MAIL FROM command allows cooperating agents in a trusted environment to communicate the authorization identity associated with individual messages.
If the server trusts the authenticated identity of the client to assert that the message was originally submitted by the supplied <mailbox>, then the server SHOULD supply the same <mailbox> in an AUTH parameter when relaying the message to any other server which supports the AUTH extension.
For this reason, servers that advertise support for this extension MUST support the AUTH parameter to the MAIL FROM command even when the client has not authenticated itself to the server.
A MAIL FROM parameter of AUTH=<> indicates that the original submitter of the message is not known. The server MUST NOT treat the message as having been originally submitted by the authenticated identity that resulted from the AUTH command.
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If the AUTH parameter to the MAIL FROM command is not supplied, the client has authenticated, and the server believes the message is an original submission, the server MAY generate a <mailbox> from the user's authenticated identity for use in an AUTH parameter when relaying the message to any server which supports the AUTH extension. The generated <mailbox> is implementation specific, but it MUST conform to the syntax of [SMTP]. If the implementation cannot generate a valid <mailbox>, it MUST transmit AUTH=<> when relaying this message.
If the server does not sufficiently trust the authenticated identity of the client, or if the client is not authenticated, then the server MUST behave as if the AUTH=<> parameter was supplied. The server MAY, however, write the value of any supplied AUTH parameter to a log file.
If an AUTH=<> parameter was supplied, either explicitly or due to the requirement in the previous paragraph, then the server MUST supply the AUTH=<> parameter when relaying the message to any server which it has authenticated to using the AUTH extension.
A server MAY treat expansion of a mailing list as a new submission, setting the AUTH parameter to the mailing list address or mailing list administration address when relaying the message to list subscribers.
Note that an implementation which is hard-coded to treat all clients as being insufficiently trusted is compliant with this specification. In that case, the implementation does nothing more than parse and discard syntactically valid AUTH parameters to the MAIL FROM command, and supply AUTH=<> parameters to any servers that it authenticates to.
An example where the original identity of the sender is trusted and known:
C: MAIL FROM:<e=mc2@example.com> AUTH=e+3Dmc2@example.com S: 250 OK
One example where the identity of the sender is not trusted or is otherwise being suppressed by the client:
C: MAIL FROM:<john+@example.org> AUTH=<> S: 250 OK
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6. Status Codes
The following error codes may be used to indicate various success or failure conditions. Servers that return enhanced status codes [ESMTP-CODES] SHOULD use the enhanced codes suggested here.
This response to the AUTH command indicates that the user needs to transition to the selected authentication mechanism. This is typically done by authenticating once using the [PLAIN] authentication mechanism. The selected mechanism SHOULD then work for authentications in subsequent sessions.
This response to the AUTH command indicates that the authentication failed due to a temporary server failure. The client SHOULD NOT prompt the user for another password in this case, and should instead notify the user of server failure.
This response to the AUTH command indicates that the selected authentication mechanism is weaker than server policy permits for that user. The client SHOULD retry with a new authentication mechanism.
This response to the AUTH command indicates that the authentication failed due to invalid or insufficient authentication credentials. In this case, the client SHOULD ask the user to supply new credentials (such as by presenting a password dialog box).
500 5.5.6 Authentication Exchange line is too long
This response to the AUTH command indicates that the authentication failed due to the client sending a [BASE64] response that is longer than the maximum buffer size available for the currently selected SASL mechanism.
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This response SHOULD be returned by any command other than AUTH, EHLO, HELO, NOOP, RSET, or QUIT when server policy requires authentication in order to perform the requested action and authentication is not currently in force.
538 5.7.11 Encryption required for requested authentication mechanism
This response to the AUTH command indicates that the selected authentication mechanism may only be used when the underlying SMTP connection is encrypted. Note that this response code is documented here for historical purposes only. Modern implementations SHOULD NOT advertise mechanisms that are not permitted due to lack of encryption, unless an encryption layer of sufficient strength is currently being employed.
This document adds several new enhanced status codes to the list defined in [ENHANCED]:
The following 3 Enhanced Status Codes were defined above:
5.7.8 Authentication credentials invalid 5.7.9 Authentication mechanism is too weak 5.7.11 Encryption required for requested authentication mechanism
This enhanced status code SHOULD be returned when the server fails the AUTH command due to the client sending a [BASE64] response which is longer than the maximum buffer size available for the currently selected SASL mechanism. This is useful for both permanent and persistent transient errors.
As described in Section 4.4 of [SMTP], an SMTP server that receives a message for delivery or further processing MUST insert the "Received:" header field at the beginning of the message content. This document places additional requirements on the content of a generated "Received:" header field. Upon successful authentication, a server SHOULD use the "ESMTPA" or the "ESMTPSA" [SMTP-TT] (when appropriate) keyword in the "with" clause of the Received header field.
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The following syntax specification uses the Augmented Backus-Naur Form notation as specified in [ABNF]. Non-terminals referenced but not defined below are as defined by [ABNF] or [SASL]. The non- terminal <mailbox> is defined in [SMTP].
Except as noted otherwise, all alphabetic characters are case- insensitive. The use of upper or lower case characters to define token strings is for editorial clarity only. Implementations MUST accept these strings in a case-insensitive fashion.
hexchar = "+" HEXDIG HEXDIG
xchar = %x21-2A / %x2C-3C / %x3E-7E ;; US-ASCII except for "+", "=", SP, and CTL
xtext = *(xchar / hexchar) ;; non-US-ASCII is only allowed as hexchar
auth-command = "AUTH" SP sasl-mech [SP initial-response] *(CRLF [base64]) [CRLF cancel-response] CRLF ;; <sasl-mech> is defined in [SASL]
auth-param = "AUTH=" xtext ;; Parameter to the MAIL FROM command. ;; This non-terminal complies with ;; syntax defined by esmtp-param [SMTP]. ;; ;; The decoded form of the xtext MUST be ;; either a <mailbox> or the two ;; characters "<>"
continue-req = "334" SP [base64] CRLF ;; Intermediate response to the AUTH ;; command. ;; This non-terminal complies with ;; syntax defined by Reply-line [SMTP].
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Security issues are discussed throughout this memo.
If a client uses this extension to get an encrypted tunnel through an insecure network to a cooperating server, it needs to be configured to never send mail to that server when the connection is not mutually authenticated and encrypted. Otherwise, an attacker could steal the client's mail by hijacking the [SMTP] connection and either pretending the server does not support the Authentication extension or causing all AUTH commands to fail.
Before the [SASL] negotiation has begun, any protocol interactions are performed in the clear and may be modified by an active attacker. For this reason, clients and servers MUST discard any knowledge obtained prior to the start of the SASL negotiation upon the establishment of a security layer.
This mechanism does not protect the TCP port, so an active attacker may redirect a relay connection attempt (i.e., a connection between two Mail Transfer Agents (MTAs)) to the submission port [SUBMIT]. The AUTH=<> parameter prevents such an attack from causing a relayed message and, in the absence of other envelope authentication, from picking up the authentication of the relay client.
A message submission client may require the user to authenticate whenever a suitable [SASL] mechanism is advertised. Therefore, it may not be desirable for a submission server [SUBMIT] to advertise a SASL mechanism when use of that mechanism grants the clients no benefits over anonymous submission.
Servers MAY implement a policy whereby the connection is dropped after a number of failed authentication attempts. If they do so, they SHOULD NOT drop the connection until at least 3 attempts to authenticate have failed.
If an implementation supports SASL mechanisms that are vulnerable to passive eavesdropping attacks (such as [PLAIN]), then the implementation MUST support at least one configuration where these SASL mechanisms are not advertised or used without the presence of an external security layer such as [TLS].
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This extension is not intended to replace or be used instead of end- to-end message signature and encryption systems such as [S/MIME] or [PGP]. This extension addresses a different problem than end-to-end systems; it has the following key differences:
1. It is generally useful only within a trusted enclave.
2. It protects the entire envelope of a message, not just the message's body.
3. It authenticates the message submission, not authorship of the message content.
4. When mutual authentication is used along with a security layer, it can give the sender some assurance that the message was successfully delivered to the next hop.
Additional security considerations are mentioned in the [SASL] specification. Additional security considerations specific to a particular SASL mechanism are described in the relevant specification. Additional security considerations for [PLAIN] over [TLS] are mentioned in Section 15 of this document.
IANA updated the entry for the "smtp" SASL protocol name to point at this document.
IANA updated the registration of the Authentication SMTP service extension as defined in Section 3 of this document. This registry is currently located at <http://www.iana.org/assignments/mail- parameters>.
[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", RFC 4234, October 2005.
[BASE64] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006.
[ESMTP-CODES] Freed, N., "SMTP Service Extension for Returning Enhanced Error Codes", RFC 2034, October 1996.
[ENHANCED] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463, January 2003.
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[ESMTP-DSN] Moore, K., "Simple Mail Transfer Protocol (SMTP) Service Extension Delivery Status Notifications (DSNs)", RFC 3461, January 2003.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[SASL] Melnikov, A. and K. Zeilenga, "Simple Authentication and Security Layer (SASL)", RFC 4422, June 2006.
[SASLprep] Zeilenga, K., "SASLprep: Stringprep Profile for User Names and Passwords", RFC 4013, February 2005.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April 2001.
[SMTP-TLS] Hoffman, P., "SMTP Service Extension for Secure SMTP over Transport Layer Security", RFC 3207, February 2002.
[StringPrep] Hoffman, P. and M. Blanchet, "Preparation of Internationalized Strings ("stringprep")", RFC 3454, December 2002.
[SUBMIT] Gellens, R. and J. Klensin, "Message Submission for Mail", RFC 4409, April 2006.
[SMTP-TT] Newman, C., "ESMTP and LMTP Transmission Types Registration", RFC 3848, July 2004.
[PLAIN] Zeilenga, K., Ed., "The PLAIN Simple Authentication and Security Layer (SASL) Mechanism", RFC 4616, August 2006.
[X509] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002.
The editors would like to acknowledge the contributions of John Myers and other contributors to RFC 2554, on which this document draws from heavily.
The editors would also like to thank Ken Murchison, Mark Crispin, Chris Newman, David Wilson, Dave Cridland, Frank Ellermann, Ned Freed, John Klensin, Tony Finch, Abhijit Menon-Sen, Philip Guenther, Sam Hartman, Russ Housley, Cullen Jennings, and Lisa Dusseault for the time they devoted to reviewing of this document and/or for the comments received.
14. Additional Requirements When Using SASL PLAIN over TLS
This section is normative for SMTP implementations that support SASL [PLAIN] over [TLS].
If an SMTP client is willing to use SASL PLAIN over TLS to authenticate to the SMTP server, the client verifies the server certificate according to the rules of [X509]. If the server has not provided any certificate, or if the certificate verification fails, the client MUST NOT attempt to authenticate using the SASL PLAIN mechanism.
After a successful [TLS] negotiation, the client MUST check its understanding of the server hostname against the server's identity as presented in the server Certificate message, in order to prevent man-in-the-middle attacks. If the match fails, the client MUST NOT attempt to authenticate using the SASL PLAIN mechanism. Matching is performed according to the following rules:
The client MUST use the server hostname it used to open the connection as the value to compare against the server name as expressed in the server certificate. The client MUST NOT use
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any form of the server hostname derived from an insecure remote source (e.g., insecure DNS lookup). CNAME canonicalization is not done.
If a subjectAltName extension of type dNSName is present in the certificate, it SHOULD be used as the source of the server's identity.
Matching is case-insensitive.
A "*" wildcard character MAY be used as the leftmost name component in the certificate. For example, *.example.com would match a.example.com, foo.example.com, etc., but would not match example.com.
If the certificate contains multiple names (e.g., more than one dNSName field), then a match with any one of the fields is considered acceptable.
1. Clarified that servers MUST support the use of the AUTH=mailbox parameter to MAIL FROM, even when the client is not authenticated.
2. Clarified the initial-client-send requirements, and give additional examples.
3. Updated references to newer versions of various specifications.
4. Required SASL PLAIN (over TLS) as mandatory-to-implement.
5. Clarified that the mechanism list can change.
6. Deprecated the use of the 538 response code.
7. Added the use of the SASLprep profile for preparing authorization identities.
8. Substantial cleanup of response codes and indicated suggested enhanced response codes. Also indicated what response codes should result in a client prompting the user for new credentials.
RFC 4954 SMTP Service Extension for Authentication July 2007
12. Added a new Enhanced Status Code for "authentication line too long" case.
13. Other general editorial clarifications.
Editors' Addresses
Robert Siemborski Google, Inc. 1600 Ampitheatre Parkway Mountain View, CA 94043, USA
Phone: +1 650 623 6925 EMail: robsiemb@google.com
Alexey Melnikov Isode Limited 5 Castle Business Village, 36 Station Road, Hampton, Middlesex, TW12 2BX, UK
EMail: Alexey.Melnikov@isode.com
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