Microsoft in Oct 2009, announced plans to release the documentation for PST file format under Microsoft’s Open Specification format, which means that anyone could implement the .pst file format “on any platform and in any tool, without concerns about patents, and without the need to contact Microsoft in any way. Now the documentation has been made publicly available. On MSDN, anyone can read through all the details about the PST Structure Specification, including all data structures it uses, examples, considerations, data algorithms, and product behavior. Armed with this info, developers will be able to interoperate with PST data in both server and client scenarios on the platform of their choice.
More info: MSDN