Open all mailboxes in a mailbox store

If you want a user Jessie to open all mailboxes in mailbox store, you grant the RECEIVE AS permission to Jessie on Mailbox Store object in Exchange System Manager.

I actually granted the following permissions to Jessie:

Read
Execute
Read Permissions
List Contents
Read Properties
List Objects
Receive AS

Jessie opens its outlook.

Jessie can open additional mailboxes in its outlook. --Jessie mailbox properties -- Advanced button.

When you grant a user has Receive As permissions to a mailbox database, that user can log in
to all mailboxes within that database, but is not able to send e-mail messages from those
mailboxes. Also, if you grant Receive As permissions at the storage group level, the
specified user can log in to all mailboxes within all databases in the storage group. For
example, you may want to grant access to the mailbox database for mobile access or for legal
review.


open only one mailbox

You grant Jessie the FULL MAILBOX ACCESS permission on the mailbox in AD.