XST File

A .xst file is an internal Microsoft Outlook template/stub file in the same binary family as .PST personal folders files. It functions as an offline store template - defining a folder hierarchy and settings before a full .ost mailbox cache is populated - and is managed automatically by Outlook or Exchange Server without user interaction.

The format uses the same PFF (Personal Folder File) binary structure as .pst and .ost files, sharing their OLE-derived container, little-endian byte ordering, and CRC32 block checksums. Two format generations exist: an older ANSI (32-bit) variant used by Outlook 97-2002, and a Unicode (64-bit) variant introduced in Outlook 2003 that supports larger file sizes and international character sets. Optional encryption levels (none, compressible, or high) match those available for .pst files.

.xst files are not user-facing. They appear during Exchange or Outlook configuration as placeholder or template files and are rarely encountered deliberately. No meaningful conversion to other formats is practical because the file represents a structural skeleton rather than actual mail data. Modern Outlook generates these files infrequently; the template functionality has largely been absorbed into the .ost creation process.

Because .xst uses the same underlying binary format as .pst, low-level forensic tools and PST viewers can parse the file structure, though they may find little usable content.