The important one is Outlook Mail, which is the modern analogue to the Hotmail interface from years ago.
This was most likely due to infrastructural differences between Windows XP and Windows Vista, which is why the two are similar, but Windows Mail can’t be used on versions prior to Vista.
Just two years later, in 2007, Windows Mail was succeeded by Windows Live Mail.
At the time, Office 365 was geared towards business and corporate users, but over time it was expanded to include regular consumers as well.
In a lot of ways, Office 365 is what Windows Live should’ve been.
Outlook debuted way back on MS-DOS — even before Windows 3.1 — but didn’t really gain traction until Outlook 97, which was packaged as part of Office 97.
With every subsequent version of Office (or simply Outlook).
Outlook Mail is the front-end while is the back-end.
As with Outlook.com, many people incorrectly refer to Outlook Mail as Outlook Online. To make it abundantly clear, Outlook Mail is the web email client while is the actual email service that Microsoft provides. Microsoft’s confusing brand strategy has also spilled into their desktop products.
Microsoft is in the process of phasing out Windows Essentials, formerly known as Windows Live Essentials.
Microsoft has included Modern-style replacements for many of the Essentials applications in Windows 8 – email, calendaring, contacts, messaging,.... As part of this movement, Microsoft planned to phase out Hotmail and replace it with a new mail system called Windows Live Mail.
As part of Microsoft’s push for Office 365, they released a collection of web apps called Outlook on the web (formerly Outlook Web App) in 2015.