Microsoft Outlook is limited in size by various versions 2003 to 2010

The file is structured as a B-tree with 512 byte nodes and leafs.[8]

Outlook 2002 and earlier use ANSI (extended ASCII with a codepage) encoding for their .pst and .ost files. This format has a maximum size of 2 GB (231 bytes) and does not support unicode. A file exceeding this size is likely to give error messages, such as ".pst has reached maximum size limit," and could become corrupted. Although superseded, this format continues to be supported by Microsoft Outlook 97 and later (98, 2000, 2002 (XP), 2003, 2007), by Internet Message Access Protocol Version 4rev1 (IMAP4) accounts and by HTTP accounts.[9]

From Outlook 2003 and onward, the standard format for .pst and .ost files is Unicode (UTF-16 little-endian). The use of 64-bit pointers instead of the 32-bit pointers of the earlier version allowed to overcome the 2 GB limit. Now, there is a user-definable maximum-file size up to 20 GB. This format is supported by Microsoft Outlook 2003 and later (2007) [9][10] A file that is created in the personal-folders format in Outlook 2003 or in Microsoft Office Outlook 2007 is not compatible with earlier versions of Microsoft Outlook and cannot be opened by using those older versions.[1] If this limit is reached or sometimes exceeded, retrieval of the .pst file can be difficult if not impossible.

As with any file, .pst files can become corrupted. Prior to Outlook 2003, the default .pst file format was ANSI and had a maximum size of 2 GB. If the .pst file were allowed to grow over 2 GB, the file would become unusable. Microsoft provides PST2GB a tool that can be used to truncate a .pst file that has grown over 2 GB. Microsoft also provides scanpst.exe, that can be used to repair other .pst file-corruption issues. In Outlook 2003 and 2007, .pst files are created in the Unicode format and have a default maximum size of 20 GB. Outlook 2010's default maximum size has increased to 50GB.[11]