IMAP Extension for Returning Mailbox METADATA in Extended LIS
Extracted elements (12)
IMAP clients that need mailbox metadata (e.g., display color) were previously forced to issue a LIST or LSUB command followed by a separate GETMETADATA command for each mailbox found, wasting bandwidth and degrading performance when requests are not pipelined. This extension collapses both operations into a single LIST command.
A mailbox that matches a LIST pattern because it has matching children but does not itself match the selection criteria will receive a LIST response but no METADATA response, as shown in the second example where 'foo' appears with CHILDINFO but without a METADATA entry.
Per RFC 5464 Section 4.4, the server may return all requested annotations in a single METADATA response per mailbox or split them across multiple METADATA responses for each mailbox; both forms are valid.
The untagged METADATA responses returned by an extended LIST command with the METADATA return option have the same syntax and semantics as those returned by GETMETADATA commands on the same set of listable mailboxes, as defined in Section 4.4.1 of RFC 5464.
For each listable mailbox matching the list pattern and selection options, the server MUST return an untagged LIST response followed by one or more untagged METADATA responses containing the mailbox annotations requested by the client.
If the server is unable to look up the annotations for a given mailbox, it MAY drop the corresponding METADATA response; in such a situation, the LIST command MUST still return a tagged OK reply.
This specification does not introduce any additional privacy concerns beyond those described in RFC 5464 (IMAP METADATA extension).
The LIST-METADATA extension adds a new RETURN option, 'METADATA', to the IMAP LIST command. The option takes one or more annotation entry names as arguments and causes the server to return METADATA responses alongside LIST responses for each matching mailbox.
IANA has registered the 'LIST-METADATA' capability in the 'IMAP Capabilities' registry. This capability string advertises server support for the METADATA return option on the LIST command.
IANA has registered the 'METADATA' option in the 'LIST-EXTENDED options' registry as a RETURN type option, with intended usage COMMON. It causes the LIST command to return METADATA responses in addition to LIST responses.
This specification does not introduce any additional security concerns beyond those described in RFC 5258 (LIST command extensions) and RFC 5464 (IMAP METADATA extension).
The ABNF extension to return-option is: return-option =/ "METADATA" SP "(" entry *(SP entry) ")", where 'return-option' is from RFC 5258 and 'entry' (an annotation entry name string) is from RFC 5464.