Microsoft Outlook has to be the most feature rich email application on the market today. It will work with any email provider and has more tools than any other on the market. I have been using Outlook since Microsoft Office 97 so I may be a bit impartial to it. I am still using the old style of Outlook as I find that the new look likes like Outlook on the web and still have not played with it enough to find the tools that I use. I will make the switch one of these days.
As you can see adding multiple accounts and using those accounts in Outlook is rather easy. Everything seems to be right where it is suppose to be. I will do an article on Thunderbird at some point as a alternative.
When working with emails outlook can handle most common forms of email. HTML and text. Some things will depend on your email provider such as recalls etc. It is easy to maintain mailboxes with categories and quick steps. It is also easy to create rules and has an integrated address book. It can also read your emails aloud. That last one can be a bit of a pain if it is a forwarded email or something. Works great on short emails though.
The Calendar app within outlook for me is better than most. It is easy to create appointments and reminders as well as adding birthdays and events.
It should be also noted that Outlook is available on Mac OS, iOS and Android. Outlook will sync across all devices that the account is logged into.