Commonly known as PST files (or Personal Storage files) they are created by users to store email data when mailbox quotas have been set by their company. Nowadays PSTs are a legal liability as they contain company information that could be used in a case (for or against the company) which may be requested through FRCP or similar laws about production of evidence, or in public bodies through the Freedom of Information directives.
Until recently there have only been a number of cheap tools around to help organizations find or retrieve PST files. Yet the problems are significant. And as with most cheap software it is so because it only tackles some of the problem or does it in a way that doesn’t capture of find everything that is needed. This is actually quite dangerous as FRCP amendments and courts are not being tolerant of only finding part of the data, and the excuse of “it was too difficult” is (rightly) being trampled on with a summary judgements being passed out.