I have reviewed this document as part of the security directorate's ongoing
effort to review all IETF documents being processed by the IESG.  These
comments were written primarily for the benefit of the security area directors.
Document editors and WG chairs should treat these comments just like any other
last call comments.

The document is short and well-written. The security considerations section is also short enough for me to copy it here:

   This specification does not introduce any additional security
   concerns beyond those described in [RFC5258].

RFC5258 is the RFC that describes LIST extensions in general. However, this document does not just describe a generic extension. It describes an extension to make the LIST command response contain the mailbox metadata. RFC 5464 which describes mailbox metadata has a Security Considerations section, and its content applies here.  So I suggest to replace "those described in [RFC5258]." with "those described in [RFC5258] and [RFC5464]."