Forwarding allows you to send mail you have received to another user. 

Select the piece of mail you want to forward, select the user or users to whom you are forwarding, and choose “Forward...” from the Snap Menu.  The Forward Mail window will appear.  Here you can add a message to the forwarded mail before sending it off.

You may also check the “Return Receipt” checkbox if you want confirmation that the forwarded mail was read.  Any file enclosures will be forwarded along with the mail unless you uncheck the “Send Enclosures” check box.  Pressing the Cmd-Enter or Cmd-Return is a shortcut for clicking on the “Send” button.