You can recover mail stored in an OST, provided that you have not tried to modify the profile that was used to create the OST file. If the profile used to create the OST file has been modified to point to a new mailbox, the information contained in the OST file may be lost.
Steps to Recover
If you have not modified your profile or attempted to log on to the new mailbox, use the following steps to copy the offline messages to a PST:
Start the Exchange Client OFFLINE.
On the Tools menu, click Services.
Add a Personal Folders (PST) service.
Select the messages that you want to copy.
On the File menu, click Copy.
In the Copy to dialog box, select the personal folders (PST) that you created in step 4.
Repeat steps 5 through 8 for each folder that contains messages.
At this point, the messages are copied to a PST. You can create a new profile and you can log on to the new server-based mailbox. After you complete this successfully, you can add a Personal Folders service (by referencing the PST created above), and you can copy the messages from the PST back to the server-based mailbox.