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