We all get a ton of emails every day. Some you can deal with right away but there are always some that you can only deal with later.  Now that you have read the email, it is no longer highlighted and it may get "lost" amongst all the other emails. (You could right-click the message and select "Mark as Unread", but there is something better.)

If you use Microsoft Outlook for your email, there is a very useful feature that will ensure that you never forget to get back to those emails:

In your email inbox is a column of little light-grey flags. If you want to "flag" an email so you can deal with it later, simply click on this flag next to the message. It will look like this:

But you get more that just pretty flags!  Outlook has now created a "task" in your "To-Do" Bar, which is normally shown as one of the boxes on your screen (as shown below) (You can move this box anywhere on the screen, and resize it etc.)

This then serves as a easy to see reminder that you need to get back to these emails.

To recall the message that you have flagged, simply Double-Click on the subject of the message in the list of Tasks. (called the "Task List" or "To-Do Bar")

If you have dealt with the message and want to "tick it off" your list - click the Red flag again, and the status will change to a "Tick" indicating that you have dealt with it.

.