In computing, a Personal Storage Table (.pst) is an open proprietary file format used to store copies of messages, calendar events, and other items within Microsoft software such as Microsoft Exchange Client, Windows Messaging, and Microsoft Outlook. The open format is controlled by Microsoft who provide free specifications and free irrevocable technology licensing. The file format may also be known as a Personal Folders (File) or Post Office File. When functioning in its capacity as a cache for Outlook's Cached Exchange Mode feature, it may be called an Off-line Storage Table (.ost) or an Off-line Folders (File). Overview In Microsoft Exchange Server, the messages, the calendar, and other data items are delivered to and stored on the server. Microsoft Outlook stores these items in a personal-storage-table (.pst) or off-line-storage-table (.ost) files that are located on the local computer. Most commonly, the .pst files are used to store archived items and the .ost files to maintain off-line availability of the items. This is an essential feature of Microsoft Outlook. The size of these files no longer counts against the size of the mailbox used; by moving files from a server mailbox to .pst files, users can free storage space on their mailservers. To use the .pst files from another location the user needs to be able to access the files directly over a network from their mail client. While it is possible to open and use a .pst file from over a network, this is unsupported, and Microsoft advises against it, as .pst files are prone to corruption when used in this manner. Both the .pst and .ost files use a fixed-block-based allocation scheme; the file is enlarged by a fixed amount of bytes, and the file internally maintains information about the allocated and non-allocated blocks. So, when data files like email messages are added to a .pst file, its file size is automatically adjusted by the mail client (if necessary). When mail is deleted from a .pst file, the size of the .pst file will stay the same, marking the space as unallocated so that it will hold future data items. Recently removed data items can actually be recovered from .pst and .ost files. To reduce the size of .pst files, the user needs to compact them. Data access Password protection can be used to protect the content of the .pst files. However, Microsoft admits that the password adds very