OS X Mail.app supports multiple sending aliases

If you want to use aliases when you send an email ...

Mail’s Email Aliases, and Complexity Hidden - Release Candidate One

... all my outgoing email appeared to come from the One True Email Address ... I looked around Mail’s account preferences for a hint as to where outgoing email aliases could be set up. Nowhere, it seemed. Could they have left that feature out? Do they want strict one-to-one mappings between incoming and outgoing addresses, and didn’t account for aliases? Surely not.

A Google search later, it turns out you can list multiple addresses separated by commas, and later those addresses will appear on a menu in the New Message window. Your selection will determine from whence that message appears to be delivered, and everybody’s happy.

This is a typical Apple move. Provide the functionality, but make it invisible and documentation free. No promises.
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