Adding an email address to OS X Mail so that you can send mail from it is simply a matter of adding it to the list of email addresses handled by your email account.
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First, select Preferences… from the Mail menu.
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The Preferences pane will open. Click on Accounts.
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Add the email address to the Email Address box. Email addresses should be separated by a comma and a space.
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Close the Preferences pane with the red bobble in the top left corner of the window. You will be prompted to save the changes. Click Save.
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You will now be able to choose which address to send mail from when composing a new message.
