The Outlook Data File (.pst) format was introduced with Microsoft Office Outlook 2003 that supports multilingual Unicode data and offers greater storage capacity for items and folders. You can determine whether your Outlook Data Files (.pst) are in the current or earlier format.
Click the File tab.
Click Account Settings, and then click Account Settings.
On the Data Files tab, select the data file that you want to verify, and then click Settings.
Check the Format box to see whether it contains the text Outlook Data File or Outlook Data File (97-2002).
Outlook Data File indicates the new format that supports multilingual Unicode data and offers greater storage capacity for items and folders. This file format is incompatible with Outlook 97-2002. Outlook Data File (97-2002) indicates the earlier format that is compatible with all versions of Outlook.