Microsoft Outlook
About
Microsoft Outlook is a somewhat ghastly but extremely popular email application which sneaks into computers as part of Microsoft Office and appeals to corporate purchasing departments by doubling as an integrated office productivity tool.
Outlook should not be confused with Microsoft Outlook Express, which has some superficial similarities but is a less powerful program designed for single users and using a different storage format.
.PST files
Outlook uses the proprietary .pst file format (see Wikipedia) whose internal structure was essentially unknown until October 2009, when Microsoft announced its intention to release technical documentation including "guidance for accessing that data from other software applications". Prior to that, there was no way to export email from Outlook without running Outlook and communicating with it through MAPI.
Since that the release of the documentation, the libpst library and utilities (official page; they are also in the apt repository for Ubuntu 10.04) have been released as open-source command-line applications for Linux.