Microsoft® Office 2003 Editions Resource Kit

Sharing Information with Earlier Versions of Outlook

In general, you can separately upgrade groups of users to Microsoft Office Outlook 2003 from earlier versions of Outlook and not lose e-mail and other message functionality between users of different versions. Outlook 2003 uses the same MAPI profiles as previous versions of Outlook. However, there are a few format changes introduced in versions of Outlook that might cause backward compatibility or interoperability issues. These few exceptions are described in the following sections.

New PST and OST file format in Outlook 2003

Outlook 2003 introduces a new Unicode file format for personal folder files (PST files) and offline folder files (OST files). The new format permits larger file sizes ...