Did you know you can edit the subject field of any email message you receive?

Open a message and type in the gray subject field, save and close.

Why would you want to edit the subject? To add keywords so you can find it easier or to make it more informative, so you don't need to open the message to see what it's about.

You can also enable in-cell editing (a custom view setting) and edit the editable fields without opening the message or item.