It is possible to use several e-mail addresses for a single mail account in Mail.app. This can be very useful, for example, when using the new Aliases feature present in .Mac, or else using different addresses multiplexed on the same account (i.e. email@example.com, firstname.lastname@example.org).
To assign multiple e-mail addresses to a single account, simply select that account in the Accounts tab of the Preferences Pane and enter all the e-mail addresses separated by commas in the “Email Address” field. Once the changes are saved, you’ll be able to choose among all addresses when composing a new mail by selecting the desired one from the “Account” drop-down box.