The MailboxMask property returns the current mailbox
wildcard mask. If no wildcard mask has been specified by the client,
this property will return an empty string.
Setting the MailboxMask property will determine which
mailboxes are returned by the Mailbox property array.
Wildcards may include the asterisk (which matches any mailbox as well
as any child mailboxes) and the percent sign (which matches any
mailbox, but does not match any child mailboxes). This property may
be used in conjunction with the MailboxPath property to
further qualify which mailboxes are returned.