Cut and Paste works a little differently in exmh than some other X programs.
To select text, hold down the first mouse button and drag the cursor from one end of the text to the other. If you drag the cursor off the top or bottom of the window, the display scrolls automatically and the selection extends. To adjust an existing selection, press the Shift key and then click or drag with the first mouse button.
Paste the selection by moving the cursor to the correct spot and pressing <Control-y>. Delete the selection by pressing <Delete>. The deleted text is saved so you can paste it into another location.
You can configure exmh to paste with the second mouse button by using the Simple Edit dialog that is found under the Bindings menu. The Sections on Editing Commands and Editing Keys describe the editing bindings in more detail.
The exmh chapters use the Tk syntax for key presses, such as <Control-y>, <Delete>, and <Tab>. This syntax is used in the Binding dialogs that let users customize key bindings. See the Section Binding User Interface.