AT&T Laboratories

200 Laurel Ave. Middletown NJ 07748 US tony@att.com

Oracle

800 Royal Oaks Monrovia CA 91016-6347 US chris.newman@oracle.com

Isode Ltd

5 Castle Business Village 36 Station Road Hampton Middlesex TW12 2BX UK Alexey.Melnikov@isode.com

EAI DSN MDN SMTP Delivery status notifications (DSNs) are critical to the correct operation of an email system. However, the existing Draft Standards (RFC 3461, RFC 3464, RFC 6522) are presently limited to ASCII text in the machine-readable portions of the protocol. This specification adds a new address type for international email addresses so an original recipient address with non-ASCII characters can be correctly preserved even after downgrading. This also provides updated content return media types for delivery status notifications and message disposition notifications to support use of the new address type. This document extends RFC 3461, RFC 3464, RFC 3798, and RFC 6522.

When an email message is transmitted using the SMTPUTF8 extension and Internationalized Email Headers, it is sometimes necessary to return that message or generate a Message Disposition Notification (MDN). As a message sent to multiple recipients can generate a status and disposition notification for each recipient, it is helpful if a client can correlate these notifications based on the recipient address it provided; thus, preservation of the original recipient is important. This specification describes how to preserve the original recipient and updates the MDN and DSN formats to support the new address types. NOTE: While this specification updates the experimental versions of this protocol by removing certain constructs (e.g., the "<addr <addr>>" address syntax is no longer permitted), the name of the Address Type "UTF-8" and the media type names message/global, message/global-delivery-status, and message/global-headers have not been changed. This specification is a revision of and replacement for . Section 6 of describes the change in approach between this specification and the previous version.

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 . The formal syntax uses the Augmented Backus-Naur Form (ABNF) notation including the core rules defined in Appendix B of and the UTF-8 syntax rules in Section 4 of .

"An Extensible Message Format for Delivery Status Notifications" defines the concept of an address type. The address format introduced in "Internationalized Email Headers" is a new address type. The syntax for the new address type in the context of status notifications is specified at the end of this section. An SMTP server that advertises both the SMTPUTF8 extension and the DSN extension MUST accept a UTF-8 address type in the ORCPT parameter including 8-bit UTF-8 characters. This address type also includes a 7-bit encoding suitable for use in a message/delivery-status body part or an ORCPT parameter sent to an SMTP server that does not advertise SMTPUTF8. This address type has 3 forms: utf&nbhy;8&nbhy;addr&nbhy;xtext, utf&nbhy;8&nbhy;addr&nbhy;unitext, and utf-8-address. Only the first form is 7-bit safe (only uses ASCII characters ). The utf-8-address form is only suitable for use in newly defined protocols capable of native representation of 8-bit characters. That is, the utf-8-address form MUST NOT be used: in the ORCPT parameter when the SMTP server doesn't advertise support for SMTPUTF8 (utf&nbhy;8&nbhy;addr&nbhy;xtext MUST be used instead); or if the SMTP server supports SMTPUTF8, but the address contains ASCII characters not permitted in the ORCPT parameter (e.g., the ORCPT parameter forbids unencoded SP and the '=' character), (either utf&nbhy;8&nbhy;addr&nbhy;unitext or utf&nbhy;8&nbhy;addr&nbhy;xtext MUST be used instead); or in a 7-bit transport environment including a message/delivery-status "Original-Recipient:" or "Final-Recipient:" field, (utf&nbhy;8&nbhy;addr&nbhy;xtext MUST be used instead). The utf-8-address form MAY be used in the ORCPT parameter when the SMTP server also advertises support for SMTPUTF8 and the address doesn't contain any ASCII characters not permitted in the ORCPT parameter. It SHOULD be used in a message/global-delivery-status "Original-Recipient:" or "Final-Recipient:" DSN field, or in an "Original-Recipient:" header field if the message is a SMTPUTF8 message. In addition, the utf&nbhy;8&nbhy;addr&nbhy;unitext form can be used anywhere where the utf-8-address form is allowed. When used in the ORCPT parameter, the UTF-8 address type requires that ASCII CTLs, SP, '\', '+', and '=' be encoded using 'unitext' encoding (see below). This is described by the utf&nbhy;8&nbhy;addr&nbhy;xtext and utf&nbhy;8&nbhy;addr&nbhy;unitext forms in the ABNF below. The 'unitext' encoding uses "\x{HEXPOINT}" syntax (EmbeddedUnicodeChar in the ABNF below) for encoding any Unicode character outside of ASCII range, as well as for encoding CTLs, SP, '\', '+', and '='. HEXPOINT is 2 to 6 hexadecimal digits. This encoding avoids the need to use the xtext encoding described in , as any ASCII characters that need to be escaped using xtext encoding never appear in any unitext-encoded string. When sending data to a SMTPUTF8-capable server, native UTF-8 characters SHOULD be used instead of the EmbeddedUnicodeChar syntax described below. When sending data to an SMTP server that does not advertise SMTPUTF8, then the EmbeddedUnicodeChar syntax MUST be used instead of UTF-8. When the ORCPT parameter is placed in a message/global-delivery-status "Original-Recipient:" field, the utf&nbhy;8&nbhy;addr&nbhy;xtext form of the UTF-8 address type SHOULD be converted to the utf-8-address form (see the ABNF below) by removing the unitext encoding. However, if an address is labeled with the UTF-8 address type but does not conform to utf-8 syntax, then it MUST be copied into the message/global-delivery-status field without alteration. The ability to encode characters with the EmbeddedUnicodeChar encodings should be viewed as a transitional mechanism and avoided when possible. It is hoped that as systems lacking support for SMTPUTF8 become less common over time, these encodings can eventually be phased out. In the ABNF below, all productions not defined in this document are defined in Appendix B of , in Section 4 of , or in .

utf-8-type-addr = "utf-8;" utf-8-enc-addr utf-8-address = Mailbox ; Mailbox as defined in [RFC6531]. utf-8-enc-addr = utf-8-addr-xtext / utf-8-addr-unitext / utf-8-address utf-8-addr-xtext = 1*(QCHAR / EmbeddedUnicodeChar) ; 7bit form of utf-8-addr-unitext. ; Safe for use in the ORCPT [RFC3461] ; parameter even when SMTPUTF8 SMTP ; extension is not advertised. utf-8-addr-unitext = 1*(QUCHAR / EmbeddedUnicodeChar) ; MUST follow utf-8-address ABNF when ; dequoted. ; Safe for using in the ORCPT [RFC3461] ; parameter when SMTPUTF8 SMTP extension ; is also advertised. QCHAR = %x21-2a / %x2c-3c / %x3e-5b / %x5d-7e ; ASCII printable characters except ; CTLs, SP, '\', '+', '='. QUCHAR = QCHAR / UTF8-2 / UTF8-3 / UTF8-4 ; ASCII printable characters except ; CTLs, SP, '\', '+' and '=', plus ; other Unicode characters encoded in UTF-8 EmbeddedUnicodeChar = %x5C.78 "{" HEXPOINT "}" ; starts with "\x" HEXPOINT = ( ( "0"/"1" ) %x31-39 ) / "10" / "20" / "2B" / "3D" / "7F" / ; all xtext-specials "5C" / (HEXDIG8 HEXDIG) / ; 2-digit forms ( NZHEXDIG 2(HEXDIG) ) / ; 3-digit forms ( NZDHEXDIG 3(HEXDIG) ) / ; 4-digit forms excluding ( "D" %x30-37 2(HEXDIG) ) / ; ... surrogate ( NZHEXDIG 4(HEXDIG) ) / ; 5-digit forms ( "10" 4*HEXDIG ) ; 6-digit forms ; represents either "\" or a Unicode code point outside ; the ASCII repertoire HEXDIG8 = %x38-39 / "A" / "B" / "C" / "D" / "E" / "F" ; HEXDIG excluding 0-7 NZHEXDIG = %x31-39 / "A" / "B" / "C" / "D" / "E" / "F" ; HEXDIG excluding "0" NZDHEXDIG = %x31-39 / "A" / "B" / "C" / "E" / "F" ; HEXDIG excluding "0" and "D"

A traditional delivery status notification comes in a three-part multipart/report container, where the first part is human-readable text describing the error, the second part is a 7-bit-only message/delivery-status, and the optional third part is used for content (message/rfc822) or header (text/rfc822-headers) return. As the present standard DSN format does not permit the return of undeliverable SMTPUTF8 messages, three new media types have been defined. ( introduced experimental versions of these media types.)

The first type, message/global-delivery-status, has the syntax of message/delivery-status with three modifications. First, the charset for message/global-delivery-status is UTF-8, and thus any field MAY contain UTF-8 characters when appropriate (see the ABNF below). In particular, the "Diagnostic-Code:" field MAY contain UTF-8 as described in SMTPUTF8; the "Diagnostic-Code:" field SHOULD be in i-default language . Second, systems generating a message/global-delivery-status body part SHOULD use the utf-8-address form of the UTF-8 address type for all addresses containing characters outside the ASCII repertoire. These systems SHOULD up-convert the utf&nbhy;8&nbhy;addr&nbhy;xtext or the utf&nbhy;8&nbhy;addr&nbhy;unitext form of a UTF-8 address type in the ORCPT parameter to the utf-8-address form of a UTF-8 address type in the "Original-Recipient:" field. Third, an optional field called "Localized-Diagnostic:" is added. Each instance includes a language tag and contains text in the specified language. This is equivalent to the text part of the "Diagnostic-Code:" field. All instances of "Localized-Diagnostic:" MUST use different language tags. The ABNF for message/global-delivery-status is specified below. In the ABNF below, all productions not defined in this document are defined in Appendix B of , in Section 4 of , or in . Note that <text-fixed> is the same as <text> from , but without <obs-text>. If or when RFC 5322 is updated to disallow <obs-text>, <text-fixed> should become just <text>. Also, if or when RFC 5322 is updated to disallow control characters in <text>, <text-fixed> should become a reference to that update instead.

utf-8-delivery-status-content = per-message-fields 1*( CRLF utf-8-per-recipient-fields ) ; "per-message-fields" remains unchanged from the definition ; in RFC 3464, except for the "extension-field", ; which is updated below. utf-8-per-recipient-fields = [ original-recipient-field CRLF ] final-recipient-field CRLF action-field CRLF status-field CRLF [ remote-mta-field CRLF ] [ diagnostic-code-field CRLF *(localized-diagnostic-text-field CRLF) ] [ last-attempt-date-field CRLF ] [ final-log-id-field CRLF ] [ will-retry-until-field CRLF ] *( extension-field CRLF ) ; All fields except for "original-recipient-field", ; "final-recipient-field", "diagnostic-code-field", ; and "extension-field" remain unchanged from ; the definition in RFC 3464. generic-address =/ utf-8-enc-addr ; Only allowed with the "utf-8" address-type. ; Updates Section 3.2.3 of RFC 3798. ; ; This indirectly updates "original-recipient-field" ; and "final-recipient-field". diagnostic-code-field = "Diagnostic-Code" ":" diagnostic-type ";" *text-fixed localized-diagnostic-text-field = "Localized-Diagnostic" ":" Language-Tag ";" *utf8-text ; "Language-Tag" is a language tag as defined in [RFC5646]. extension-field =/ extension-field-name ":" *utf8-text ; Updates Section 7 of RFC3798 text-fixed = %d1-9 / ; Any ASCII character except for NUL, %d11 / ; CR, and LF. %d12 / ; See note above about <text-fixed> %d14-127 utf8-text = text-fixed / UTF8-non-ascii UTF8-non-ascii = UTF8-2 / UTF8-3 / UTF8-4

The second type, used for returning the content, is message/global, which is similar to message/rfc822, except it contains a message with UTF-8 headers. This media type is described in .

The third type, used for returning the headers, is message/global-headers and contains only the UTF-8 header fields of a message (all lines prior to the first blank line in a SMTPUTF8 message). Unlike message/global, this body part provides no difficulties for the present infrastructure.

Note that as far as a multipart/report container is concerned, message/global-delivery-status, message/global, and message/global-headers MUST be treated as equivalent to message/delivery-status, message/rfc822, and text/rfc822-headers. That is, implementations processing multipart/report MUST expect any combinations of the 6 media types mentioned above inside a multipart/report media type. All three new types will typically use the "8bit" Content-Transfer-Encoding. (In the event all content is 7-bit, the equivalent traditional types for delivery status notifications MAY be used. For example, if information in a message/global-delivery-status part can be represented without any loss of information as message/delivery-status, then the message/delivery-status body part may be used.) Note that relaxed a restriction from MIME regarding the use of Content-Transfer-Encoding in new "message" subtypes. This specification explicitly allows the use of Content-Transfer-Encoding in message/global-headers and message/global-delivery-status. This is not believed to be problematic as these new media types are intended primarily for use by newer systems with full support for 8-bit MIME and UTF-8 headers.

If an SMTP server that advertises both SMTPUTF8 and DSN needs to return an undeliverable SMTPUTF8 message, then it has two choices for encapsulating the SMTPUTF8 message when generating the corresponding multipart/report: If the return-path SMTP server does not support SMTPUTF8, then the undeliverable body part and headers MUST be encoded using a 7-bit Content-Transfer-Encoding such as "base64" or "quoted-printable" , as detailed in . Otherwise, "8bit" Content-Transfer-Encoding can be used.

Message Disposition Notifications have a similar design and structure to DSNs. As a result, they use the same basic return format. When generating an MDN for a UTF-8 header message, the third part of the multipart/report contains the returned content (message/global) or header (message/global-headers), same as for DSNs. The second part of the multipart/report uses a new media type, message/global-disposition-notification, which has the syntax of message/disposition-notification with two modifications. First, the charset for message/global-disposition-notification is UTF-8, and thus any field MAY contain UTF-8 characters when appropriate (see the ABNF below). (In particular, the failure-field, and the error-field MAY contain UTF-8. These fields SHOULD be in i-default language .) Second, systems generating a message/global-disposition-notification body part (typically a mail user agent) SHOULD use the UTF-8 address type for all addresses containing characters outside the ASCII repertoire. The MDN specification also defines the "Original-Recipient:" header field, which is added with a copy of the contents of ORCPT at delivery time. When generating an "Original-Recipient:" header field, a delivery agent writing a UTF-8 header message in native format SHOULD convert the utf&nbhy;8&nbhy;addr&nbhy;xtext or the utf&nbhy;8&nbhy;addr&nbhy;unitext form of a UTF-8 address type in the ORCPT parameter to the corresponding utf-8-address form. The MDN specification also defines the "Disposition-Notification-To:" header field, which is an address header field and thus follows the same 8-bit rules as other address header fields such as "From:" and "To:" when used in a UTF-8 header message.

; ABNF for "original-recipient-header", "original-recipient-field", ; and "final-recipient-field" from RFC 3798 is implicitly updated ; as they use the updated "generic-address" as defined in ; Section 4 of this document. failure-field = "Failure" ":" *([FWS] utf8-text) ; "utf8-text" is defined in Section 4 of this document. error-field = "Error" ":" *([FWS] utf8-text) ; "utf8-text" is defined in Section 4 of this document.

This specification does not create any new IANA registries. However, the following items have been registered as a result of this document.

The mail address type registry was created by . The registration template response follows: The address-type name. UTF-8 The syntax for mailbox addresses of this type, specified using BNF, regular expressions, ASN.1, or other non-ambiguous language. See . If addresses of this type are not composed entirely of graphic characters from the ASCII repertoire, a specification for how they are to be encoded as graphic ASCII characters in an "Original-Recipient:" or "Final-Recipient:" DSN field. This address type has 3 forms (as defined in ): utf&nbhy;8&nbhy;addr&nbhy;xtext, utf&nbhy;8&nbhy;addr&nbhy;unitext, and utf-8-address. Only the first form is 7-bit safe.

The mail diagnostic type registry was created by and updated by . This specification replaces . The registration for the 'smtp' diagnostic type has been updated to reference RFC &rfc.number; in addition to and to remove the reference to . When the 'smtp' diagnostic type is used in the context of a message/delivery-status body part, it remains as presently defined. When the 'smtp' diagnostic type is used in the context of a message/global-delivery-status body part, the codes remain the same, but the text portion MAY contain UTF-8 characters.

message global-headers none none This media type contains Internationalized Email Headers with no message body. Whenever possible, the 8-bit content transfer encoding SHOULD be used. When this media type passes through a 7-bit-only SMTP infrastructure, it MAY be encoded with the base64 or quoted-printable content transfer encoding. See . It is important that this media type is not converted to a charset other than UTF-8. As a result, implementations MUST NOT include a charset parameter with this media type. Although it might be possible to down-convert this media type to the text/rfc822-header media type, such conversion is discouraged as it loses information. RFC &rfc.number; SMTPUTF8 servers and email clients that support multipart/report generation or parsing. none In the event this is saved to a file, the extension ".u8hdr" is suggested. The 'TEXT' type code is suggested as files of this type are typically used for diagnostic purposes and suitable for analysis in a UTF-8-aware text editor. A uniform type identifier (UTI) of "public.utf8&nbhy;email&nbhy;message&nbhy;header" is suggested. This type conforms to "public.utf8&nbhy;plain&nbhy;text" and "public.plain&nbhy;text". See the Authors' Addresses section of this document. COMMON This media type contains textual data in the UTF-8 charset. It typically contains octets with the 8th bit set. As a result, a transfer encoding is required when a 7-bit transport is used. See the Authors' Addresses section of this document. IETF Standards Process

message global-delivery-status none none This media type contains delivery status notification attributes in the UTF-8 charset. The 8-bit content transfer encoding MUST be used with this content-type, unless it is sent over a 7-bit transport environment, in which case quoted-printable or base64 may be necessary. See This media type provides functionality similar to the message/delivery-status content-type for email message return information. Clients of the previous format will need to be upgraded to interpret the new format; however, the new media type makes it simple to identify the difference. RFC &rfc.number; SMTP servers and email clients that support delivery status notification generation or parsing. none The extension ".u8dsn" is suggested. A uniform type identifier (UTI) of "public.utf8-email-message-delivery-status" is suggested. This type conforms to "public.utf8-plain-text". See the Authors' Addresses section of this document. COMMON This is expected to be the second part of a multipart/report. See the Authors' Addresses section of this document. IETF Standards Process

message global-disposition-notification none none This media type contains disposition notification attributes in the UTF-8 charset. The 8-bit content transfer encoding MUST be used with this content-type, unless it is sent over a 7-bit transport environment, in which case quoted-printable or base64 may be necessary. See . This media type provides functionality similar to the message/disposition-notification content-type for email message disposition information. Clients of the previous format will need to be upgraded to interpret the new format; however, the new media type makes it simple to identify the difference. RFC &rfc.number; Email clients or servers that support message disposition notification generation or parsing. none The extension ".u8mdn" is suggested. A uniform type identifier (UTI) of "public.utf8-email-message-disposition-notification" is suggested. This type conforms to "public.utf8-plain-text". See the Authors' Addresses section of this document. COMMON This is expected to be the second part of a multipart/report. See the Authors' Addresses section of this document. IETF Standards Process

Automated use of report types without authentication presents several security issues. Forging negative reports presents the opportunity for denial-of-service attacks when the reports are used for automated maintenance of directories or mailing lists. Forging positive reports may cause the sender to incorrectly believe a message was delivered when it was not. Malicious users can generate report structures designed to trigger coding flaws in report parsers. Report parsers need to use secure coding techniques to avoid the risk of buffer overflow or denial-of-service attacks against parser coding mistakes. Code reviews of such parsers are also recommended. Malicious users of the email system regularly send messages with forged envelope return paths, and these messages trigger delivery status reports that result in a large amount of unwanted traffic on the Internet. Many users choose to ignore delivery status notifications because they are usually the result of "blowback" from forged messages and thus never notice when messages they sent go undelivered. As a result, support for correlation of delivery status and message disposition notification messages with sent messages has become a critical feature of mail clients and possibly mail stores, if the email infrastructure is to remain reliable. In the short term, simply correlating Message-IDs may be sufficient to distinguish true status notifications from those resulting from forged originator addresses. But in the longer term, including cryptographic signature material that can securely associate the status notification with the original message is advisable. As this specification permits UTF-8 in additional fields, the security considerations of UTF-8 apply.

American National Standards Institute (formerly United States of America Standards Institute) ANSI X3.4-1968 has been replaced by newer versions with slight modifications, but the 1968 version remains definitive for the Internet. TWNIC Microsoft Oracle

Because the warning disposition modifier was previously removed in RFC 3798, warning-field has also been removed from this specification. Other minor corrections.

TBD