Follow below mentioned steps to perform successful restoration of mailbox from old backup.

1. On exchange 2007 server create registry key as mentioned below

\HKEY_LOCAL_MACHINE\SOFTWARE\Symantec\BackupExec\Engine\ESE

Create DWORD (32 Bit)  value named "e2k7_mbox" and set value as "1"

2. Restart the "Remote Agent Service" on exchange server

3. On backup server, enable legacy mailbox support from following location

Tools --> Options --> Exchange -->  

4. Restart BackupExec services from backup server

5. Create test mailbox on exchange 2007 server

6. Configure restore job on backup server, select desired mailbox in selection list (from old backup)

7. In Exchange redirect restoration type exchange 2007 server name in server box

8. Select "Redirect mailboxes or public folders" and enter the test mailbox name (you created) in "Restore to mailbox"

9. Once restoration completed revert the changes made (by following these procedures)

Hope this helps you to restore your mailbox.