If you have an e-mail account that has more than one address associated with it, for example the messages for firstname.lastname@example.org and email@example.com might both come to the same inbox. In OS X Mail, you don’t have to set up multiple accounts for each of these addresses to choose one as the From address on outgoing mail.
Instead, choose Mail: Preferences, click on the Accounts tab, and select the account. Type the multiple addresses in the Email Address field, separated by commas. Now, before sending a message, you can choose the From address you want to use from the pop-up Account menu.