Locate and then click the mailbox store that you want. For example, to locate the Mailbox Store (My_Server) store, follow these steps:
Expand Administrative Groups, expand First Administrative Group, expand Servers, and then expand My_Server.
Expand First Storage Group, and then click Mailbox Store (My_Server).
On the Action menu, click Properties, and then click the Limits tab.
Under Deletion settings, type a number that is less than the age of the orphaned mailbox in the Keep deleted mailboxes for (days) box. For a mailbox that is newly orphaned, type 0 (zero).
Configure the maintenance schedule. To do this, follow these steps:
Click the Database tab.
In the Maintenance interval box, click Use custom schedule, and then click Customize.
In the list of days, click the current day so that the row for that day is highlighted. This schedules the maintenance for all day.

Note The scheduling of maintenance for all day can be set or removed. To do this, click a day in the list of days.
Click OK.
Exit Exchange System Manager.
Allow for online maintenance to finish. This may take several hours. You will see event ID 1221 logged in the Application log when online maintenance is finished.
In Exchange System Manager, use the Run Cleanup Agent on the Mailboxes folder that contains the orphaned mailbox.

/Chris