Microsoft’s Paul Lorimer, Group Manager for Microsoft Office Interoperability, has announced that the .pst file format will be published:

In order to facilitate interoperability and enable customers and vendors to access the data in .pst files on a variety of platforms, we will be releasing documentation for the .pst file format. This will allow developers to read, create, and interoperate with the data in .pst files in server and client scenarios using the programming language and platform of their choice.

The initials .pst stand, I believe, for “Personal store”. This is the format used by standalone Outlook, for users without Exchange. You can also have Exchange deliver email to a .pst, though more normally Exchange mail is stored in an .ost (“Offline store”) which replicates the mailbox on the server. The .pst format is still used for archiving in this scenario. I’m not sure how different .pst and .ost are internally, or whether Microsoft intends to document both.

Any move towards open formats is welcome, though I’m not sure how important this one is; further, Outlook is frail enough as it is, so I’m nervous about third-party software modifying a .pst and perhaps getting it slightly wrong and causing problems. Programmatic access to Outlook data has long been available, via the ancient MAPI or via Outlook’s COM API.

In just slightly related news, my help post on the error message Cannot open the Outlook window is the most viewed post on this site this month and the fourth most viewed last month; it has 129 comments.

I suspect most users would prefer a faster and more robust Outlook over and above a published file format; unlike Office document formats, a .pst is not generally shared with others.