What Does Outlook Express (OE) Mean?
Outlook Express (OE) is an email client that lets the user save, send, receive and manage email messages. Downloading and installing Internet Explorer versions 4.0 through 6.0 included Outlook Express for free. Starting with Windows 98, Outlook Express was the default email client until it was replaced by Windows Mail in Windows Vista and later, in Windows 7, by Windows Live Mail.
Techopedia Explains Outlook Express (OE)
Outlook Express was a part of Internet Explorer in earlier versions of Windows. It was later offered as standalone software. Outlook Express is not to be confused with Microsoft Outlook — they both perform the function of sending and receiving emails, but their codebase is entirely different from each other and they are, indeed, separate programs. Their similar names often caused confusion about them being the same.
The last Outlook Express email client edition was released in Windows XP, after which it was discontinued and replaced by Windows Mail and was integrated with Windows Messenger.