Flagging emails you need to follow up on has been around almost forever, but Outlook takes them a step further. Turn flags into tasks, set due dates, mark as completed. Master Outlook flags.
I have looked at turning emails into tasks with Outlook before and that article is worth reading, especially as it covers Outlook flagged emails on phones. However, you can now do even more with Outlook and tasks and it is worth revisiting the topic.
I will be using Outlook in a web browser on a Windows PC for this, but it works just the same in a browser on macOS and also in the free Outlook Mac app.
Flag emails in Outlook
There is more than one way to flag emails in Outlook, so use whichever one is most convenient.

Move the mouse over an email in the inbox and icons appear on it. Click the flag icon to mark the message. If you don’t see a flag icon, go to Quick Actions in Outlook settings and there are options to add various action buttons.

Another way to flag emails is to click the three dots in the top right corner of a message. A menu is displayed and Flag is one of the options.
Set a due date
Another way to add a flag to an email in Outlook is to click the icon in the toolbar at the top. The benefit of this is that it has an additional function and clicking the little v arrow at the right of it displays a menu.

Flagging an email turns it into a task and using this menu, you can set a date by which it must be completed. Select Today, Tomorrow, This week or Next week.
Another way to set a due date for the email task is by right clicking the flag icon in the inbox. Click to set the flag and then right click to set a due date.

Notice that there are options on these menus to set No date and Mark as complete. The latter is needed if you set a due date.
View Outlook flagged emails
How do you check Outlook flagged emails and see due dates when you need to take action? Just select an email in the inbox and at the top of the email it says Flag for follow up. Start by… Due by… It can be today, tomorrow, this week or next week.

How do you find flagged emails when they are buried among all your other emails? Click the Filter button and then select Flagged. Only flagged emails are then shown in the inbox and you can quickly go through them and see when they are due.

There is also another way and flagging an email adds it to Microsoft To Do as a task with a due date. Open Microsoft To Do app or website and select Flagged email in the sidebar to list all flagged emails and see due dates.

You can set them as completed here in To Do, but you can also set them as completed in Outlook mail. Mark complete is on the flag menu (shown in screenshots 2 and 3).

When a flagged email is marked as complete, a tick appears next to it in the inbox. This makes it easy to see which emails you have followed up on and those that are still to do.
Are you using Outlook flagged emails and setting dates for taking action on them?