It’s always mainly about Excel, but some people are really set on Outlook, too.
Set up Thunderbird on my work desktop that day, and haven't looked back. Wish I would have way sooner, to be honest.
The interface is far nicer than outlook, but the performance is just too annoying for me.
The order varies depending on what you do, each can become absolutely critical.
Outlook online doesn't seem to have any rule capability and can't talk to 3rd party email servers.