Suppose you sent a message to a bunch of people and you realize you forgot someoneor someone tells you they did not get the message. Instead of opening the message in your Sent mailbox, copying its contents, and pasting it into a new message, just select the message, choose Message from the menu bar -> Send Again (Command-Shift-D), and replace the existing recipients with the new one(s), or the person who did not get the message. The subject and message contents remain the same as before (although you can edit them if you like).
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