There are really two ways that you can accomplish this.

Method 1:

Unhide the mailbox/user account, wait a few minutes for replication,
then add the account under the Account options (Tools, E-mail Accounts,
Change, Advanced, Add additional mailboxes). This would for obvious
reasons also allow you to send a message quite easily to the now
unhidden account. The only exception here is if you are using Outlook
2003 with cached mode and using the Offline address book, it would
require a rebuild of the offline address book (and re-download of OAB)
in order to get it working.

Once you have added the account to Outlook, you can then re-hide the account.

Method 2:

Use the legacyExchangeDN attribute of the account to add the mailbox.
Pre-Exchange 2000, you did this by adding the actual Distinguished Name
of the mailbox, which could be obtained by opening the Exchange Admin
program in raw mode and obtaining the DN of the object. Details of
doing that are documented in http://support.microsoft.com/?kbid=142781.

With Exchange 2000 and newer, this capability still exists, but instead
of using the DN, you use the legacyExchangeDN. Note that using the full
Distinguished Name of the object will not work for Exchange 2000 and
newer - it must be the legacyExchangeDN. Use the same procedure as
Method 1 (Tools, E-mail accounts, etc.) and when prompted for the
mailbox, enter the full legacyExchangeDN. You will notice that it is
then changed to the display name of the hidden object. This process
works the same when addressing a message to a hidden recipient. Simply
paste in the legacyExchangeDN, hit Ctrl-K, and the name will be resolved
to the display name of the hidden user.

How do you get the legacyExchangeDN value, you ask? Easy. Use ADSIEdit
(part of the Windows 2000/2003 support tools), or use your favorite LDAP
client (ldp.exe is either part of the support tools, or part of the
Resource Kit) and list the attributes of the hidden user.

Note that the above instructions assume that you are adding a hidden
mailbox as an "additional" mailbox, but the procedure should work the
same for setting up a new profile.

- posted by Ben Winzenz @ 1:05 PM