Sub-mailbox folder hierarchy to represent a separate physical mailbox to a user

Architecture that provides an abstraction of a mailbox store that allows clients to connect to multiple alternate mailboxes associated with a single user, exposing the same mechanism used to connect to a user's main mailbox. Beneath this abstraction, support for alternate mailboxes is implemented as a folder hierarchy contained within the user's own mailbox or another physical mailbox. The architecture uses a sub-mailbox folder hierarchy to virtualize a separate physical mailbox. On top of this basic infrastructure, other features can be built such as server-hosted e-mail archives and mailboxes mirrored from other e-mail and PIM data systems. In support thereof, a mechanism is provided for an abstraction layer to expose all or a subset of the alternate mailbox features, and to advertise which alternate mailbox features are supported per alternate mailbox.