Many IMAP clients (such as Microsoft Outlook and Outlook Express) and AirSync clients (such as Windows Mobile, Apple iPhone, Nokia PDAs, etc.) cannot handle foreign Mailboxes directly, and they cannot use the Account Mailbox subscription to access foreign Mailboxes.
Mailbox Aliases can be used to let these clients access foreign Mailboxes.
A Mailbox Alias is a name associated with some [foreign] Mailbox name. For example, you can create a Mailbox Alias salesBox for the ~sales/INBOX Mailbox name. You will see the salesBox Mailbox in your client application, but in reality this will be the INBOX Mailbox in the sales Account.
Mailbox Aliases can be created only on the topmost level of the Account Mailbox hierarchy, that means that the Mailbox Alias name cannot contain the slash (/) symbol.
A Mailbox Alias provides access to the Mailbox it is associated with, and to all its sub-mailboxes (subject to Access Control List restrictions).
A Mailbox Alias can contain just a foreign Account name (~accountName). Such an Alias provides access to all accessible Mailboxes in that foreign Account. The Mailbox Alias itself is presented as an unselectable Mailbox name.
The owner of the Account chief has granted "lookup" and other access rights for his Mailboxes INBOX and Pending to the assistant Account.
The user assistant has created the Mailbox Alias boss pointing to ~j.smith.
When the user assistant connects to her Account using any IMAP or XIMSS client, or the WebUser Interface, she sees all her own Mailboxes, the unselectable Mailbox boss, and also the boss/INBOX and boss/Pending Mailboxes.
If the user j.smith creates a new Mailbox Urgent in his Account and grants access rights for that Mailbox to the assistant Account, the user assistant will immediately see the new Mailbox as the boss/Urgent Mailbox.