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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.

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:

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:

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:

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 : 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:

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:

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 Informative References

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.