What was the name of Microsoft's email client included with Internet Explorer 4?

Microsoft's email client included with Internet Explorer 4 was Outlook Express.

Outlook Express 4.0 was integrated with Internet Explorer 4 and appeared in the Windows 98 era. It was designed for everyday internet email and newsgroups, rather than the full business-management role associated with Microsoft Outlook. Its message store used mailbox files with the .mbx extension.

The name is a frequent source of confusion. Outlook Express and Outlook were separate Microsoft products: Outlook was part of Microsoft Office and combined email with calendars, contacts, tasks, and scheduling, while Outlook Express was a lighter client distributed with Windows and Internet Explorer. Windows Mail was its later successor in Windows Vista, not the Internet Explorer 4 client.

Outlook Express 5 later changed its storage system to separate .dbx files for mailbox folders. The product was eventually replaced by Windows Mail and later Windows Live Mail, before Microsoft moved users toward newer mail apps and webmail services.