Outlook Express 4.x uses two file types, MBX and IDX, to store your messages on your machine. For instance, you would find an inbox.mbx and outbox.mbx on your system where your e-mail messages exist, along with corresponding IDX files. Should you create additional folders to better organize your mail, Outlook Express will actually create two files, "folderX.mbx" and "folderX.idx". The first file is where the actual messages will be stored; the latter contains indexing information for all the messages that will be stored in the new folder. The "X" is an ordinal number, which corresponds to the folder creation order -- for example, our first folder created would be folder1.mbx, then folder2.mbx and so on; each with its corresponding IDX file.
Normally, these files are kept in the installation directory; for users of Windows NT:
C:\Winnt\Profiles\"user name"\Application Data\Microsoft\Outlook Express\Mail
and for users of Windows 95 or Windows 98:
C:\Program Files\Outlook Express\"username"\mail
If the file isn't in that location, it may have been installed in another area. In that case, simply click the button in the "Repair" page to find the file.