Sieve Email Filtering: delivery by mailboxid

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Table of Contents

1. 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 multi-step 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 extending fileinto to reference an immutable mailboxid, sieve rules can continue to target the same mailbox, even if it gets renamed.¶

2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. Sieve capability string

The server advertises the capability "mailboxid", and scripts which use the following extensions MUST explicitly request the capability "mailboxid".¶

Example:¶

require "mailboxid";

4. 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 [RFC2342] with the specified [RFC8474] 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 with the objectid of the mailbox to file into.¶

Example:¶

require "fileinto";
require "mailboxid";

if header :contains ["from"] "coyote" {
    fileinto :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3"
             "INBOX.harassment";
}

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.

require "fileinto";
require "mailboxid";
require "special-use";
if header :contains ["from"] "coyote" {
    fileinto :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3"
             :specialuse "\\Junk"
             "INBOX.harassment";
}

require "fileinto";
require "mailboxid";
require "mailbox";
require "special-use";

fileinto :mailboxid "F1234567"
         :specialuse "\\Archive"
         :create
         "INBOX.Archive";
            # creates INBOX.Archive with use \Archive but
            # with a different mailboxid.

FCC =/ [":mailboxid" <mailboxid: string>]

require ["enotify", "fcc", "mailboxid"];
notify :fcc "INBOX.Sent"
       :mailboxid "F6352ae03-b7f5-463c-896f-d8b48ee3"
       :message "You got mail!"
       "mailto:ken@example.com";

5. Test ":mailboxidexists"

The "mailboxidexists" test is true if all mailboxes listed in the "mailboxids" argument exist in the mailstore, and each allows the user in whose context the Sieve script runs to "deliver" messages into it. When the mailstore is an IMAP server, "delivery" of messages is possible if:¶

a) the READ-WRITE response code is present for the mailbox (see Section 7.1 of [RFC3501]), if IMAP Access Control List (ACL) [RFC4314] is not supported by the server, or¶

b) the user has 'p' or 'i' rights for the mailbox (see Section 5.2 of [RFC4314]).¶

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 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.harassment";
    } else {
        fileinto "INBOX.harassment";
    }
}

Note to implementers: this test behaves identically to the mailboxexists test defined in [RFC5490] but operates on mailboxids rather than mailbox names.¶

6. 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 [RFC5229] is enabled.¶

7. Formal Syntax

test /= "mailboxidexists" string-list

tag /= ":mailboxid" string

If [I-D.ietf-extra-sieve-fcc] is supported:¶

FCC =/ [":mailboxid" <mailboxid: string>]

8. Security considerations

Because mailboxid is always generated by the server, implementations MUST NOT allow sieve to make an endrun around this protection by creating mailboxes with the specified ID by using ":create" and ":mailboxid" in a fileinto rule for a non-existant mailbox.¶

Implementers are referred to the security considerations sections of [RFC5228] and [RFC8474].¶

9. IANA considerations

IANA are requested to add a capability to the sieve-extensions registry:¶

To: iana@iana.org
Subject: Registration of new Sieve extension

Capability name: mailboxid
Description: adds a test for checking mailbox existence by objectid,
             and new optional arguments to fileinto and :fcc which
             allow selecting the destination mailbox by objectid.
RFC number: this RFC
Contact address: The EXTRA discussion list <extra@ietf.org>

10. Acknowledgements

This document borrows heavily from [RFC5490] for the matching mailboxexists test, and from [I-D.ietf-extra-sieve-special-use] for an example of modifying the fileinto command.¶

Thanks to Ned Freed and Ken Murchison and Alexey Melnikov for feedback on the EXTRA mailing list.¶

11. Changes

(EDITOR: remove this section before publication)¶

Author's Address