Fastmail

Level 2 114 William St Melbourne VIC 3000 Australia brong@fastmailteam.com https://www.fastmail.com

Applications EXTRA sieve email The OBJECTID capability of IMAP (RFC 8474) allows clients to identify mailboxes by a unique identifier that survives renaming. This document extends the Sieve email filtering language (RFC 5228) to allow using that same unique identifier as a target for fileinto rules and for testing the existence of mailboxes.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

• .  Introduction
• .  Conventions Used in This Document
• .  Sieve Capability String
• .  Argument :mailboxid to Command fileinto
  • .  Interaction with Mailbox Extension
  • .  Interaction with Special-Use Extension
• .  Interaction with FCC Extension
• .  Test mailboxidexists
• .  Interaction with Variables Extension
• .  Security Considerations
• .  IANA Considerations
• . References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Author's Address

Introduction Sieve rules are sometimes created using graphical interfaces, which allow users to select the mailbox to be used as a target for a rule. If that mailbox is renamed, the client may also update its internal representation of the rule and update the Sieve script to match; however, this is a multistep process and subject to partial failures. Also, if the folder is renamed by a different mechanism (e.g., another IMAP client), the rules will get out of sync. By telling fileinto to reference the immutable MAILBOXID specified by , using the extension specified herein, Sieve rules can continue to target the same mailbox, even if it gets renamed.

Conventions Used in This Document 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.

Sieve Capability String Scripts that use the extensions defined in this document MUST explicitly require the capability "mailboxid". Example: require "mailboxid";

Argument :mailboxid to Command fileinto Normally, the fileinto command delivers the message in the mailbox specified using its positional mailbox argument. However, if the optional :mailboxid argument is also specified, the fileinto command first checks whether a mailbox exists in the user's personal namespace with the specified MAILBOXID . If a matching mailbox is found, that mailbox is used for delivery. If there is no such mailbox, the fileinto action proceeds as it would without the :mailboxid argument. The tagged argument :mailboxid to fileinto consumes one additional token, a string containing the OBJECTID of the target mailbox. Example: require "fileinto"; require "mailboxid"; if header :contains ["from"] "coyote" { fileinto :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3" "INBOX.harassment"; }

Interaction with Mailbox Extension For servers that also support the mailbox extension defined in , if both the :create and :mailboxid arguments are provided to a fileinto command and no matching mailbox is found, then a new mailbox will be created. This new mailbox will have the name specified by the positional mailbox argument (); however, it will get a different MAILBOXID (chosen by the server) rather than the one specified by the :mailboxid argument to fileinto. Example: require "fileinto"; require "mailboxid"; require "mailbox"; fileinto :mailboxid "Fnosuch" :create "INBOX.no-such-folder"; # creates INBOX.no-such-folder, but it doesn't # get the "Fnosuch" mailboxid.

Interaction with Special-Use Extension For servers that also support delivery to special-use mailboxes , it is an error to specify both :mailboxid and :specialuse in the same fileinto command. Advanced filtering based on both special-use and MAILBOXID can be built with explicit specialuse_exists and mailboxidexists tests.

Interaction with FCC Extension This document extends the definition of the :fcc argument defined in so that it can optionally be used with the :mailboxid argument. The syntax for FCC is extended here using ABNF : MAILBOXID-OPT = ":mailboxid" objectid FCC-OPTS =/ MAILBOXID-OPT If the optional :mailboxid argument is specified with :fcc, it instructs the Sieve interpreter to check whether a mailbox exists with the specific MAILBOXID. If such a mailbox exists, the generated message is filed into that mailbox. Otherwise, the generated message is filed into the :fcc target mailbox. As with fileinto, it is an error to specify both :mailboxid and :specialuse for the same fcc rule. Example: require ["enotify", "fcc", "mailboxid"]; notify :fcc "INBOX.Sent" :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3" :message "You got mail!" "mailto:ken@example.com";

Test mailboxidexists Usage: mailboxidexists <mailbox-objectids: string-list> The mailboxidexists test is true if every string argument provided is the MAILBOXID of a mailbox that exists in the mailstore and that allows the user in whose context the Sieve script runs to deliver messages into it. When the mailstore is an IMAP server that also supports IMAP Access Control List (ACL) , delivery is allowed if the user has the 'p' or 'i' rights for the mailbox (see ). When the mailstore is an IMAP server that does not support IMAP ACL, delivery is allowed if the READ-WRITE response code is present for the mailbox when selected by the user (see ). Note that a successful mailboxidexists test for a mailbox doesn't necessarily mean that a "fileinto :mailboxid" action on this mailbox would succeed. For example, the fileinto action might put the user over quota. The mailboxidexists test only verifies existence of the mailbox and whether the user in whose context the Sieve script runs has permissions to execute fileinto on it. Example: require "fileinto"; require "mailboxid"; if header :contains ["from"] "coyote" { if mailboxidexists "F6352ae03-b7f5-463c-896f-d8b48ee3" { fileinto :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3" "INBOX.name.will.not.be.used"; } else { fileinto "INBOX.harassment"; } }

Interaction with Variables Extension There is no special interaction defined; however, as an OBJECTID is a string in this document, OBJECTID values can contain variable expansions if is enabled.

Security Considerations Because MAILBOXID is always generated by the server, implementations MUST NOT allow Sieve to make an end run around this protection by creating mailboxes with the specified ID by using :create and :mailboxid in a fileinto rule for a nonexistent mailbox. Implementers are referred to the Security Considerations sections of and .

IANA Considerations IANA has added the following capability to the "Sieve Extensions" registry at :

Capability name:

mailboxid

Description:

adds a test for checking mailbox existence by OBJECTID and new optional arguments to fileinto and :fcc that allow selecting the destination mailbox by OBJECTID.

RFC number:

RFC 9042

Contact address:

EXTRA discussion list <extra@ietf.org>

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. 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] This document describes a language for filtering email messages at time of final delivery. It is designed to be implementable on either a mail client or mail server. It is meant to be extensible, simple, and independent of access protocol, mail architecture, and operating system. It is suitable for running on a mail server where users may not be allowed to execute arbitrary programs, such as on black box Internet Message Access Protocol (IMAP) servers, as the base language has no variables, loops, or ability to shell out to external programs. [STANDARDS-TRACK] Internet technical specifications often need to define a formal syntax. Over the years, a modified version of Backus-Naur Form (BNF), called Augmented BNF (ABNF), has been popular among many Internet specifications. The current specification documents ABNF. It balances compactness and simplicity with reasonable representational power. The differences between standard BNF and ABNF involve naming rules, repetition, alternatives, order-independence, and value ranges. This specification also supplies additional rule definitions and encoding for a core lexical analyzer of the type common to several Internet specifications. [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. This document updates RFC 3501 (IMAP4rev1) with persistent identifiers on mailboxes and messages to allow clients to more efficiently reuse cached data when resources have changed location on the server. The Sieve email filtering language provides a number of action commands, some of which can generate additional messages on behalf of the user. This document defines an extension to such commands to allow a copy of any generated message to be filed into a target mailbox. This document updates RFCs 5230 and 5435 by adding a new tagged argument to the Vacation and Notify actions, respectively. Informative References 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] 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] In advanced mail filtering rule sets, it is useful to keep state or configuration details across rules. This document updates the Sieve filtering language (RFC 5228) with an extension to support variables. The extension changes the interpretation of strings, adds an action to store data in variables, and supplies a new test so that the value of a string can be examined. [STANDARDS-TRACK] This memo defines an extension to the Sieve mail filtering language (RFC 5228) for accessing mailbox and server annotations, checking for mailbox existence, and controlling mailbox creation on "fileinto" action. [STANDARDS-TRACK] The SPECIAL-USE capability of the IMAP protocol (RFC 6154) allows clients to identify special-use mailboxes, e.g., where draft or sent messages should be put. This simplifies client configuration. In contrast, the Sieve mail filtering language (RFC 5228) currently has no such capability. This memo defines a Sieve extension that fills this gap: it adds a test for checking whether a special-use attribute is assigned for a particular mailbox or any mailbox, and it adds the ability to file messages into a mailbox identified solely by a special-use attribute.

Acknowledgements This document borrows heavily from for the matching mailboxexists test and from for an example of modifying the fileinto command. Thanks to , , and for feedback on the EXTRA mailing list.

Author's Address Fastmail

Level 2 114 William St Melbourne VIC 3000 Australia brong@fastmailteam.com https://www.fastmail.com