Outlook Tasks
I use Outlook for email and because of that I ended up using the Tasks feature which turns out to be pretty damn useful.
It is called the To-Do Bar and if you don’t immediately see it you can open it by clicking on View->To-Do Bar. Select “Normal” and make sure the “Task List” is checked in that list.
When a work email comes in that needs attending to, I usually Flag it by clicking the flag icon to the right of each email in my inbox.