http://www.ietf.org/rfc/rfc2342.txt
IMAP4 [RFC-2060] does not define a default server namespace. As a result, two common namespace models have evolved:

The "Personal Mailbox" model, in which the default namespace that is presented consists of only the user's personal mailboxes. To access shared mailboxes, the user must use an escape mechanism to reach another namespace.

The "Complete Hierarchy" model, in which the default namespace that is presented includes the user's personal mailboxes along with any other mailboxes they have access to.

These two models, create difficulties for certain client operations. This document defines a NAMESPACE command that allows a client to discover the prefixes of namespaces used by a server for personal mailboxes, other users' mailboxes, and shared mailboxes. This allows a client to avoid much of the manual user configuration that is now necessary when mixing and matching IMAP4 clients and servers.