Select one of the messages among those you have received that you'd like to answer. Press and hold the first mouse button over the Reply... menu button. A menu with a few entries will be displayed. Select the Reply to sender menu entry by dragging the mouse down to that entry and releasing the mouse button. You can also press r instead of using the menu.
The built-in editor opens a window with a message that has the header initialized based on the header fields from the message to which you are replying. The editor positions the cursor at the begining of the message body. You can enter your reply message as you did with the previous messages. You should also double-check the header fields. You can add yourself to the Cc: field to get a copy of your reply. When you are done, click the Send button in the editor window to send the message.
The Section Replying to Mail with repl describes a number of ways to control the format of your reply messages -- but note that repl -query doesn't work with exmh. The Section Resources for Buttons shows you how to define new Reply... menu entries for variations on reply.
It should not take long for you to get the copy of the reply message. Wait a minute or so and click the Inc button, or press i.