Many IMAP4 [RFC-2060] clients present to the user a hierarchical view
of the mailboxes that a user has access to. Rather than initially
presenting to the user the entire mailbox hierarchy, it is often
preferable to show to the user a collapsed outline list of the
mailbox hierarchy (particularly if there is a large number of
mailboxes). The user can then expand the collapsed outline hierarchy
as needed. It is common to include within the collapsed hierarchy a
visual clue (such as a "+") to indicate that there are child
mailboxes under a particular mailbox. When the visual clue is
clicked the hierarchy list is expanded to show the child mailboxes.
Several IMAP vendors implemented this proposal, and it is proposed to
document this behavior and functionality as an Informational RFC.
There is interest in addressing the general extensibility of the IMAP
LIST command through an IMAP LIST Extension draft. Similar
functionality to the \HasChildren and \HasNoChildren flags could be
incorporated into this new LIST Extension. It is proposed that the
more general LIST Extension draft proceed on the standards track with
this proposal being relegated to informational status only.
If the functionality of the \HasChildren and \HasNoChildren flags
were incorporated into a more general LIST extension, this would have
the advantage that a client could then have the opportunity to
request whether or not the server should return this information.
This would be an advantage over the current draft for servers where
this information is expensive to compute, since the server would only
need to compute the information when it knew that the client
requesting the information was able to consume it.