To read a message move your mouse over the sender's subject (the blue underline text under subject). The mouse pointer will change to a hand:
Click once and the message will open.
From this window you can reply, delete, move and perform various functions. These are described further on in this document.
Note: Between the Date and the Subject columns is a small column which is unlabelled. If you see a small unopened envelope, this means that the mail is unread. An opened envelope indicates when the message has been read; and an opened envelope with an arrow pointing to the left or right indicates that the message has been replied to or forwarded.