1. Open the Reply form via either of 3 ways:
The first way
Tick the checkbox corresponding to the message you want to reply in the mails list pane, then click 
on the action bar.
The second way
Right-click the message you want to reply, then select Reply from the drop-down menu.
The third way
Click the message that you want to reply to display its content, then select at the upper right corner of the message content.
2. Compose the content in the Compose New Message window.
3. Click 
to send your reply.
Reply to all recipients of the message
1. Tick the checkbox corresponding to the message you want to reply, then click 
on the Action bar;
Or right-click the message, then select Reply to All from the drop-down menu.
2. Compose your reply message in the
Compose New Message
window, then click to send your reply.