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Applications EXTRA IMAP This specification extends the Internet Message Access Protocol (IMAP4rev1, RFC 3501) to support UTF-8 encoded international characters in user names, mail addresses, and message headers. This specification replaces RFC 6855. This specification does not extend IMAP4rev2 , since that protocol includes everything in this extension.

Introduction This specification forms part of the Email Address Internationalization protocols described in the Email Address Internationalization Framework document . It extends IMAP [RFC3501}} to permit UTF-8 in headers, as described in "Internationalized Email Headers" . It also adds a mechanism to support mailbox names using the UTF-8 charset. This specification creates two new IMAP capabilities to allow servers to advertise these new extensions. This specification assumes that the IMAP server will be operating in a fully internationalized environment, i.e., one in which all clients accessing the server will be able to accept non-ASCII message header fields and other information, as specified in Section 3. At least during a transition period, that assumption will not be realistic for many environments; the issues involved are discussed in Section 7 below. This specification replaces an earlier, experimental approach to the same problem as well as .

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

"UTF8=ACCEPT" IMAP Capability and UTF-8 in IMAP Quoted-Strings The "UTF8=ACCEPT" capability indicates that the server supports the ability to open mailboxes containing internationalized messages with the "SELECT" and "EXAMINE" commands, and the server can provide UTF-8 responses to the "LIST" and "LSUB" commands. This capability also affects other IMAP extensions that can return mailbox names or their prefixes, such as NAMESPACE and ACL . The "UTF8=ONLY" capability, described in Section 6, implies the "UTF8=ACCEPT" capability. A server is said to support "UTF8=ACCEPT" if it advertises either "UTF8=ACCEPT" or "UTF8=ONLY". A client MUST use the "ENABLE" command with the "UTF8=ACCEPT" option (defined in Section 4 below) to indicate to the server that the client accepts UTF-8 in quoted-strings and supports the "UTF8=ACCEPT" extension. The "ENABLE UTF8=ACCEPT" command is only valid in the authenticated state. The IMAP base specification forbids the use of 8-bit characters in atoms or quoted-strings. Thus, a UTF-8 string can only be sent as a literal. This can be inconvenient from a coding standpoint, and unless the server offers IMAP non-synchronizing literals , this requires an extra round trip for each UTF-8 string sent by the client. When the IMAP server supports "UTF8=ACCEPT", it supports UTF-8 in quoted-strings with the following syntax: UTF8-3 = UTF8-4 = ]]> When this extended quoting mechanism is used by the client, the server MUST reject, with a "BAD" response, any octet sequences with the high bit set that fail to comply with the formal syntax requirements of UTF-8 . The IMAP server MUST NOT send UTF-8 in quoted-strings to the client unless the client has indicated support for that syntax by using the "ENABLE UTF8=ACCEPT" command. If the server supports "UTF8=ACCEPT", the client MAY use extended quoted syntax with any IMAP argument that permits a string (including astring and nstring). However, if characters outside the US-ASCII repertoire are used in an inappropriate place, the results would be the same as if other syntactically valid but semantically invalid characters were used. Specific cases where UTF-8 characters are permitted or not permitted are described in the following paragraphs. All IMAP servers that support "UTF8=ACCEPT" SHOULD accept UTF-8 in mailbox names, and those that also support the Mailbox International Naming Convention described in RFC 3501, Section 5.1.3, MUST accept UTF8-quoted mailbox names and convert them to the appropriate internal format. Mailbox names MUST comply with the Net-Unicode Definition (, Section 2) with the specific exception that they MUST NOT contain control characters (U+0000-U+001F and U+0080-U+ 009F), a delete character (U+007F), a line separator (U+2028), or a paragraph separator (U+2029). Once an IMAP client has enabled UTF-8 support with the "ENABLE UTF8=ACCEPT" command, it MUST NOT issue a "SEARCH" command that contains a charset specification. If an IMAP server receives such a "SEARCH" command in that situation, it SHOULD reject the command with a "BAD" response (due to the conflicting charset labels).

"APPEND" Command If the server supports "UTF8=ACCEPT", then the server accepts UTF-8 headers in the "APPEND" command message argument. If an IMAP server supports "UTF8=ACCEPT" and the IMAP client has not issued the "ENABLE UTF8=ACCEPT" command, the server MUST reject, with a "NO" response, an "APPEND" command that includes any 8-bit character in message header fields.

"LOGIN" Command and UTF-8 This specification does not extend the IMAP "LOGIN" command to support UTF-8 usernames and passwords. Whenever a client needs to use UTF-8 usernames or passwords, it MUST use the IMAP "AUTHENTICATE" command, which is already capable of passing UTF-8 usernames and credentials. Although using the IMAP "AUTHENTICATE" command in this way makes it syntactically legal to have a UTF-8 username or password, there is no guarantee that the user provisioning system utilized by the IMAP server will allow such identities. This is an implementation decision and may depend on what identity system the IMAP server is configured to use.

"UTF8=ONLY" Capability The "UTF8=ONLY" capability indicates that the server supports "UTF8=ACCEPT" (see Section 4) and that it requires support for UTF-8 from clients. In particular, this means that the server will send UTF-8 in quoted-strings, and it will not accept the older international mailbox name convention (modified UTF-7 ). Because these are incompatible changes to IMAP, explicit server announcement and client confirmation is necessary: clients MUST use the "ENABLE UTF8=ACCEPT" command before using this server. A server that advertises "UTF8=ONLY" will reject, with a "NO [CANNOT]" response , any command that might require UTF-8 support and is not preceded by an "ENABLE UTF8=ACCEPT" command. IMAP clients that find support for a server that announces "UTF8=ONLY" problematic are encouraged to at least detect the announcement and provide an informative error message to the end-user. Because the "UTF8=ONLY" server capability includes support for "UTF8=ACCEPT", the capability string will include, at most, one of those and never both. For the client, "ENABLE UTF8=ACCEPT" is always used -- never "ENABLE UTF8=ONLY".

Dealing with Legacy Clients In most situations, it will be difficult or impossible for the implementer or operator of an IMAP (or POP) server to know whether all of the clients that might access it, or the associated mail store more generally, will be able to support the facilities defined in this document. In almost all cases, servers that conform to this specification will have to be prepared to deal with clients that do not enable the relevant capabilities. Unfortunately, there is no completely satisfactory way to do so other than for systems that wish to receive email that requires SMTPUTF8 capabilities to be sure that all components of those systems -- including IMAP and other clients selected by users -- are upgraded appropriately. When a message that requires SMTPUTF8 is encountered and the client does not enable UTF-8 capability, choices available to the server include hiding the problematic message(s), creating in-band or out-of-band notifications or error messages, or somehow trying to create a surrogate of the message with the intention of providing useful information to that client about what has occurred. Such surrogate messages cannot be actual substitutes for the original message: they will almost always be impossible to reply to (either at all or without loss of information) and the new header fields or specialized constructs for server-client communications may go beyond the requirements of current email specifications (e.g., ). Consequently, such messages may confuse some legacy mail user agents (including IMAP clients) or not provide expected information to users. There are also trade-offs in constructing surrogates of the original message between accepting complexity and additional computation costs in order to try to preserve as much information as possible (for example, in "Post-Delivery Message Downgrading for Internationalized Email Messages" ) and trying to minimize those costs while still providing useful information (for example, in "Simplified POP and IMAP Downgrading for Internationalized Email" ). Implementations that choose to perform downgrading SHOULD use one of the standardized algorithms provided in RFC 6857 or RFC 6858. Getting downgrade algorithms right, and minimizing the risk of operational problems and harm to the email system, is tricky and requires careful engineering. These two algorithms are well understood and carefully designed. Because such messages are really surrogates of the original ones, not really "downgraded" ones (although that terminology is often used for convenience), they inevitably have relationships to the originals that the IMAP specification did not anticipate. This brings up two concerns in particular: First, digital signatures computed over and intended for the original message will often not be applicable to the surrogate message, and will often fail signature verification. (It will be possible for some digital signatures to be verified, if they cover only parts of the original message that are not affected in the creation of the surrogate.) Second, servers that may be accessed by the same user with different clients or methods (e.g., POP or webmail systems in addition to IMAP or IMAP clients with different capabilities) will need to exert extreme care to be sure that UIDVALIDITY behaves as the user would expect. Those issues may be especially sensitive if the server caches the surrogate message or computes and stores it when the message arrives with the intent of making either form available depending on client capabilities. Additionally, in order to cope with the case when a server compliant with this extension returns the same UIDVALIDITY to both legacy and "UTF8=ACCEPT"-aware clients, a client upgraded from being non-"UTF8=ACCEPT"-aware MUST discard its cache of messages downloaded from the server. The best (or "least bad") approach for any given environment will depend on local conditions, local assumptions about user behavior, the degree of control the server operator has over client usage and upgrading, the options that are actually available, and so on. It is impossible, at least at the time of publication of this specification, to give good advice that will apply to all situations, or even particular profiles of situations, other than "upgrade legacy clients as soon as possible".

Issues with UTF-8 Header Mailstore When an IMAP server uses a mailbox format that supports UTF-8 headers and it permits selection or examination of that mailbox without issuing "ENABLE UTF8=ACCEPT" first, it is the responsibility of the server to comply with the IMAP base specification and the Internet Message Format with respect to all header information transmitted over the wire. The issue of handling messages containing non-ASCII characters in legacy environments is discussed in Section 7.

IANA Considerations the "IMAP 4 Capabilities" registry contains a number of references to RFC6855. IANA, please change them to point to this document.

Security Considerations The security considerations of UTF-8 and SASLprep apply to this specification, particularly with respect to use of UTF-8 in usernames and passwords. Otherwise, this is not believed to alter the security considerations of IMAP. Special considerations, some of them with security implications, occur if a server that conforms to this specification is accessed by a client that does not, as well as in some more complex situations in which a given message is accessed by multiple clients that might use different protocols and/or support different capabilities. Those issues are discussed in Section 7.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. The Internet Message Access Protocol, Version 4rev1 (IMAP4rev1) allows a client to access and manipulate electronic mail messages on a server. IMAP4rev1 permits manipulation of mailboxes (remote message folders) in a way that is functionally equivalent to local folders. IMAP4rev1 also provides the capability for an offline client to resynchronize with the server. IMAP4rev1 includes operations for creating, deleting, and renaming mailboxes, checking for new messages, permanently removing messages, setting and clearing flags, RFC 2822 and RFC 2045 parsing, searching, and selective fetching of message attributes, texts, and portions thereof. Messages in IMAP4rev1 are accessed by the use of numbers. These numbers are either message sequence numbers or unique identifiers. IMAP4rev1 supports a single server. A mechanism for accessing configuration information to support multiple IMAP4rev1 servers is discussed in RFC 2244. IMAP4rev1 does not specify a means of posting mail; this function is handled by a mail transfer protocol such as RFC 2821. [STANDARDS-TRACK] ISO/IEC 10646-1 defines a large character set called the Universal Character Set (UCS) which encompasses most of the world's writing systems. The originally proposed encodings of the UCS, however, were not compatible with many current applications and protocols, and this has led to the development of UTF-8, the object of this memo. UTF-8 has the characteristic of preserving the full US-ASCII range, providing compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values. This memo obsoletes and replaces RFC 2279. This document describes how to prepare Unicode strings representing user names and passwords for comparison. The document defines the "SASLprep" profile of the "stringprep" algorithm to be used for both user names and passwords. This profile is intended to be used by Simple Authentication and Security Layer (SASL) mechanisms (such as PLAIN, CRAM-MD5, and DIGEST-MD5), as well as other protocols exchanging simple user names and/or passwords. [STANDARDS-TRACK] Most IMAP extensions are used by the client when it wants to and the server supports it. However, a few extensions require the server to know whether a client supports that extension. The ENABLE extension allows an IMAP client to say which extensions it supports. [STANDARDS-TRACK] The Internet today is in need of a standardized form for the transmission of internationalized "text" information, paralleling the specifications for the use of ASCII that date from the early days of the ARPANET. This document specifies that format, using UTF-8 with normalization and specific line-ending sequences. [STANDARDS-TRACK] This document specifies the Internet Message Format (IMF), a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This specification is a revision of Request For Comments (RFC) 2822, which itself superseded Request For Comments (RFC) 822, "Standard for the Format of ARPA Internet Text Messages", updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs. [STANDARDS-TRACK] Full use of electronic mail throughout the world requires that (subject to other constraints) people be able to use close variations on their own names (written correctly in their own languages and scripts) as mailbox names in email addresses. This document introduces a series of specifications that define mechanisms and protocol extensions needed to fully support internationalized email addresses. These changes include an SMTP extension and extension of email header syntax to accommodate UTF-8 data. The document set also includes discussion of key assumptions and issues in deploying fully internationalized email. This document is a replacement for RFC 4952; it reflects additional issues identified since that document was published. [STANDARDS-TRACK] Internet mail was originally limited to 7-bit ASCII. MIME added support for the use of 8-bit character sets in body parts, and also defined an encoded-word construct so other character sets could be used in certain header field values. However, full internationalization of electronic mail requires additional enhancements to allow the use of Unicode, including characters outside the ASCII repertoire, in mail addresses as well as direct use of Unicode in header fields like "From:", "To:", and "Subject:", without requiring the use of complex encoded-word constructs. This document specifies an enhancement to the Internet Message Format and to MIME that allows use of Unicode in mail addresses and most header field content. This specification updates Section 6.4 of RFC 2045 to eliminate the restriction prohibiting the use of non-identity content-transfer- encodings on subtypes of "message/". [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Informative References The Internet Message Access Protocol [STANDARDS-TRACK] This document defines a NAMESPACE command that allows a client to discover the prefixes of namespaces used by a server for personal mailboxes, other users' mailboxes, and shared mailboxes. [STANDARDS-TRACK] The Access Control List (ACL) extension (RFC 2086) of the Internet Message Access Protocol (IMAP) permits mailbox access control lists to be retrieved and manipulated through the IMAP protocol. This document is a revision of RFC 2086. It defines several new access control rights and clarifies which rights are required for different IMAP commands. [STANDARDS-TRACK] 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. [STANDARDS-TRACK] This specification extends the Internet Message Access Protocol version 4rev1 (IMAP4rev1) to support UTF-8 encoded international characters in user names, mail addresses, and message headers. This document defines an Experimental Protocol for the Internet community. This specification extends the Internet Message Access Protocol (IMAP) to support UTF-8 encoded international characters in user names, mail addresses, and message headers. This specification replaces RFC 5738. The Email Address Internationalization (SMTPUTF8) extension to SMTP allows Unicode characters encoded in UTF-8 and outside the ASCII repertoire in mail header fields. Upgraded POP and IMAP servers support internationalized messages. If a POP or IMAP client does not support Email Address Internationalization, a POP or IMAP server cannot deliver internationalized messages to the client and cannot remove the message. To avoid that situation, this document describes a mechanism for converting internationalized messages into the traditional message format. As part of the conversion process, message elements that require internationalized treatment are recoded or removed, and receivers are able to recognize that they received messages containing such elements, even if they cannot process the internationalized elements. This document specifies a method for IMAP and POP servers to serve internationalized messages to conventional clients. The specification is simple, easy to implement, and provides only rudimentary results. This document specifies a protocol for clients to efficiently query, fetch, and modify JSON-based data objects, with support for push notification of changes and fast resynchronisation and for out-of- band binary data upload/download. The Internet Message Access Protocol Version 4rev2 (IMAP4rev2) allows a client to access and manipulate electronic mail messages on a server. IMAP4rev2 permits manipulation of mailboxes (remote message folders) in a way that is functionally equivalent to local folders. IMAP4rev2 also provides the capability for an offline client to resynchronize with the server. IMAP4rev2 includes operations for creating, deleting, and renaming mailboxes; checking for new messages; removing messages permanently; setting and clearing flags; parsing per RFCs 5322, 2045, and 2231; searching; and selective fetching of message attributes, texts, and portions thereof. Messages in IMAP4rev2 are accessed by the use of numbers. These numbers are either message sequence numbers or unique identifiers. IMAP4rev2 does not specify a means of posting mail; this function is handled by a mail submission protocol such as the one specified in RFC 6409.

Appendix A. Design Rationale This non-normative section discusses the reasons behind some of the design choices in this specification. The "UTF8=ONLY" mechanism simplifies diagnosis of interoperability problems when legacy support goes away. In the situation where backwards compatibility is not working anyway, the non-conforming "just-send-UTF-8 IMAP" has the advantage that it might work with some legacy clients. However, the difficulty of diagnosing interoperability problems caused by a "just-send-UTF-8 IMAP" mechanism is the reason the "UTF8=ONLY" capability mechanism was chosen.

Appendix B. Acknowledgments The authors wish to thank the participants of the EAI working group for their contributions to this document, with particular thanks to Harald Alvestrand, David Black, Randall Gellens, Arnt Gulbrandsen, Kari Hurtta, John Klensin, Xiaodong Lee, Charles Lindsey, Alexey Melnikov, Subramanian Moonesamy, Shawn Steele, Daniel Taharlev, and Joseph Yee for their specific contributions to the discussion.

Appendix B. Changes since RFC 6855 This non-normative section describes the changes made since . This document removes APPEND's UTF8 data item, making the UTF8-related syntax compatible with IMAP4rev2 as defined by and making it simpler for clients to support IMAP4rev1 and IMAP4rev2 with the same code. IMAP4rev2 provides roughly the same abilities as but does not include APPEND's UTF8 item. None of , IMAP4rev2 or JMAP specify any way to learn whether a particular message was stored using the UTF8 data item. As of today, an IMAP client cannot learn whether a particular message was stored using the UTF8 data item, nor would it be able to trust that information even if IMAP4rev1/2 were extended to provide that information. In July 2023, one of the authors found only one IMAP client that uses the UTF8 data item, and that client uses it incorrectly (it sends the data item for all messages if the server supports UTF8=ACCEPT, without regard to whether a particular message includes any UTF8 at all). For these reason, it was judged best to revise and adopt the same syntax as IMAP4rev2.