Microsoft Exchange gained wide usage with the release of Windows 95, as this was the only mail client that came bundled with it. In 1996, it was renamed to Windows Messaging, because of the upcoming release of Microsoft Exchange Server, and continued to be included throughout later releases of Windows up until the initial release of Windows 98, which by then included Outlook Express 4.0 as the default mail client.
The Windows Messaging email client had two branches of successors:
In software bundled with Windows itself, these were Internet Mail and News in Windows 95 (and bundled with Internet Explorer 3), which was succeeded by Outlook Express 4.0 in Windows 98 (bundled with Internet Explorer 4.0 in Windows 95) and throughout newer Windows systems. These did not use the .pst file type.
Microsoft Outlook became the professional-grade and more direct successor of MS Exchange Client, which still uses the .pst file type.
Microsoft Fax, also called Microsoft at Work Fax (AWF), is the fax component to provide Send-and-Receive Fax capability; sent and received faxes were stored in the same .pst file as other messages, first attempt of unified messaging by Microsoft; also the ability to act as fax server, which is not available in later versions of Windows until Windows Vista.
Because Microsoft Outlook used the same basic Windows Messaging profile, account and e-mail settings (MAPI), MS Exchange users not familiar with it could have been led into thinking that Outlook created a double profile and that it made copies of all their mail while they were just checking to see what the new MS Outlook (ver. 97) looked like. This way some MS Exchange users could have unknowingly deleted all their e-mail that they perceived to be 'double', as MS Outlook did not have any front-end feature to notify users that it was actually using the same MS Exchange or Windows Messaging account.
In a similar fashion, e-mail that did not use traditional message formatting, was shown the same way: actual message content was delivered in the form of text attachments with the *.ATT extension, which could be opened through Notepad. These files were in turn saved in the active Temp directory and some sensitive e-mail could therefore have been made available for other users to see.
HTML e-mail was shown in such a way that the message contained an *.ATT or *.htm attachment, which had to be saved and then viewed in a browser, as MS Exchange did not have support for HTML-formatted messages.
International characters were unsupported. Some e-mail that was sent with a non-ASCII or non-7/8-bit character set was shown in the form of text attachments, which had to be saved and then read in a web browser, with the browser's text encoding set for a specified code page.