new standard format for .pst and .ost files is Unicode (UTF-16 little-endian), with 64-bit pointers instead of 32-bit to allow larger than 2 GiB sizes. The limit became 20 GB for Outlook 2003-2007, and increased to 50 GB from Outlook 2010. A file that is created in the personal-folders format in Outlook 2003 or later is not compatible and cannot be opened by earlier versions. As with any file, .pst files can become corrupted. Growth over the limit has been a consistent problem; ANSI .pst that grew beyond 2 GiB and Unicode .pst that grew beyond 20 or 50 GB would become unusable. The scanpst.exe tool has been included with all versions of Outlook to detect and repair .pst database corruption, and Microsoft at one point offered a special-purpose PST2GB that would simply truncate the file to 2 GiB to allow Outlook to repair it. Entourage and Outlook for Mac Microsoft Entourage was Microsoft's email and personal information program for Mac OS X (replaced by Outlook for Macintosh in Microsoft Office for Mac 2011). While superficially similar to Outlook, it was an entirely different application, and used a unique database format which cannot be imported or exported, though user data can be imported and exported to and from another unique format called .rge (a bundle consisting of many individual files plus metadata). Entourage 2008, the current version as of May 2010, has no support for .pst files, though there exists Microsoft's .pst import tool for Entourage 2004; however, the tool could only import .pst files from Outlook for Mac 2001, and not any Windows versions. Entourage's replacement, Outlook for Office 2011 for Intel Macs, was able to import Outlook .pst files from Windows; however, data will be stored as many individual files, rather than in a single database such as .pst or the Entourage database. Outlook for Mac 2001, which runs under Mac OS 9 or the Mac OS X Classic Environment, connects exclusively to Exchange servers, and to this day is closer to its Windows counterpart than Entourage is. It works directly with 'Outlook 97-2002' .pst files, and can freely interchange those files with Outlook for Windows, as recent versions are still compatible with the older .pst format. The limit for Outlook 2011 is 10GB. References External links Microsoft has released some of the format specification and reference implementation. Email storage formats Windows architecture Microsoft Office Freeware