Outlook users will be familiar with the PST file extension. This is a Microsoft Outlook Storage file, where all your email information, as well as your calendars and contacts are stored. In older versions of Outlook (Outlook 97 through Outlook 2000) the PST file was encoded using an ANSI text format which only allowed a maximum size of 1.9 GB.
If this size limit was reached, mail retrieval and even deletion of mail items became difficult if not impossible. In newer version of Outlook (Outlook 2003 and 2007 currently) support increased mail capacity defaulted to 20 GB using a new encoding process known as Unicode. The Archive folder however, still uses the older format, but this issue has been corrected since Service Pack 1.
Outlook users that have an Exchange server have the option of using a local PST file in addition to an OST file. This file allows you to keep an offline copy of your email stored on your local computer (so you can get to it when the server is not available) but keeps the mail on the server, which allows for greater security as well as retrieval using Outlook Web Access (OWA), a way to remotely view your mail using a standard web browser. The OST file also has the file limitation of 1.9GB. However, this
restriction can be lifted with the appropriate patches installed on the Exchange Server
Since both file types store email, they can be backed up on a regular basis simply by copying them to the server or a shared location on a daily or weekly basis.
While it is possible to store and access them from the server or shared location, this is not supported by Microsoft and will lead to trouble later on since the file requires quick read/write response times.