The Mailboxes Settings page allows you to set the Mailbox Subscription - the list of your own and foreign Mailboxes you want to use.
You can open the Mailboxes Settings page using the link on the Settings page:
Type a Mailbox name into an empty field and click the Update button to add a Mailbox to the subscription list.
To specify a Foreign Mailbox, type the Tilda sign (~), the user name, the slash sign (/) and then the Mailbox name. Make sure that user has already granted you the Select access right for that Mailbox.