As many users know, you can edit the subject line in a message. This makes it easier to find messages especially when they don't include a subject.
If you are new to editing the subject, simply open the message, type in the subject field and save. We have more information and a video tutorial at Change the Subject of an Incoming Message.
In older versions of Outlook the Message list shows the edited subject:
In Outlook 2010 and newer, when you use the default view with the Date (Conversation) setting, you'll see the original subject in the message list:
As you can see, the edited subject shows in the preview pane but not in the message list, because you are only changing the subject, not the conversation title, and the new Conversation view uses the Conversation field.
When you turn off the Show messages in conversation view, Outlook will display the edited subject, just as it does in older versions of Outlook.
Compact Header in Outlook 2013 and up
Outlook 2013 SP1 and newer allow users to collapse the message header to a more compact size. When the header is collapsed, the subject field is NOT editable.
Edit Subject and Disable Conversation View Tutorial
To turn off Show messages in conversation either:
Switch to the View ribbon and deselect the Show in Conversation option
Right click on the row of field names above the message list and click Show message in conversation to deselect it.
Changes can't be Saved error message
When you edit a subject field of a message that contains an attachment and the attachment is open, you'll receive an error message:
Your changes cannot be saved because you don't have permission to modify some or all of the items in this folder. Do you want to save a copy of this item in the default folder for the item?
To avoid this, you need to close the attachment first, then save and close the message with the edited subject field.