MicrosoftCorporation's Email Client.

The Windows version was fondly known by many as "Outbreak Express" because of its nasty habit of executing programs without users realising it, or making it far too easy for users to do it to themselves.

The version for MacOS (pre-OSX) was a very different program, with some nice features like an easily-accessible "View Source" function to view the raw RFC2822 format of a message, as well as AppleScript? support which included functions for importing/exporting messages.