Once you have opened Mail.app go to the sent items and select the e-mail you want to resend. Don’t open the message just select it. Now head up to the Message tab on the menu bar and select Send Again or if you are a big Keyboard Shortcut user hit “Shift + Command + D”. A new window will pop up and your original message will be there just like it was just before you hit send. As a matter of good practice you should check to make sure the e-mail address is correct just in case the reason you are resending it is because the person did not get the e-mail. This would also help if you left someone off an e-mail you are sending to a group of people. Just change the”TO:” addresses and send it again.