By default, Outlook places each Offline Folders (.ost) file and Personal Folders (.pst) file that it creates in the %userprofile%\Local Settings\Application Data\Microsoft\Outlook folder. This setting allows you to change the default behavior and store the files in any folder.
Open your registry and find the appropriate key below.
Create a new REG_EXPAND_SZ (Expandable String) value called 'ForcePSTPath' and set it to equal the full path of the required personal folder directory (e.g. "C:\Mailbox").
Restart Outlook for the change to take effect.
Note: This is a expandable string value so you can use environment variables, such as %userprofile%, to specify the path. When the value does not exist the default value of "%userprofile%\Local Settings\Application Data\Microsoft\Outlook" is used.
Value Name: ForcePSTPath
Data Type: REG_EXPAND_SZ (Expanded String Value)
Value Data: Full path to personal folder directory