The SubscribeMailbox method adds the specified mailbox to
the current user's list of active or subscribed mailboxes. The user
will remain subscribed to the mailbox across multiple sessions, until
the UnsubscribeMailbox method is called to remove the mailbox
from the subscription list.
Note that if a user subscribes to a mailbox and that mailbox is
later renamed or deleted, the mailbox will not be automatically
removed from the user's subscription list. To determine if the
current mailbox is in the user's subscription list, check the
Subscribed property.