Unread email messages will have an Unread icon in the side bar. In order to mark an email as read or unread, you can do the following:
mark an individual email or a complete email thread as read or unread.
mark all email messages in an email folder as unread
How to mark an email as unread:
Select a read email in the side bar.
Click on Mark unread in the display area.
In order to mark this email as read, click on Mark read.
Tip: You can also mark multiple email messages at once or all the email messages in an email thread as being read.
How to mark all email messages in a folder as read:
Select an email folder in the folder tree.
Click the Folder-specific actions icon next to the folder name. Click on Mark all email messages as read.