Microsoft Outlook is described as 'Manage emails, calendars, contacts, and tasks with an intuitive interface. Features smart inbox, file management, and third-party app integration' and is a very popular Email Client in the office & productivity category. There are more than 100 alternatives to Microsoft Outlook for a variety of platforms, including Windows, Mac, Linux, Android and iPhone apps. The best Microsoft Outlook alternative is Thunderbird, which is both free and Open Source. Other great apps like Microsoft Outlook are K-9 Mail, Evolution, eM Client and Betterbird.
Despite its age, Thunderbird remains the most fully featured email client out there. Various add-ons can extend its functionality, it (now) has a built in calendar and, unlike Outlook, it's free, open source and respects your privacy.