How to get Mail.app to support multiple "personalities"

As any inbox warrior would tell you, there's an easy secret to simplifying inbox mayhem: only have one inbox. As easy as that seems, there's often a pull to set up new ones. For example, you set up a new domain, or you start working on a project "for" an organization and want to appear like you're sending from that organization's domain. "That's easy," you say! Just use personalities. For us Mail.app users on the Mac, that was easier said than done. For all of its greatness, Mail.app didn't (to my knowledge) support personalities. That was, until today... I'm addicted to Mail.app, for its simplicity and for its speed, and for its integration with the OS. It just keeps getting better. But what about personalities, where are they? Well, like so many incredible -- yet hidden -- OS X features, it was there all along, for those "in the know."

Here's the recipe I found on the Mac Rumors forum:

Use multiple email addresses by separating them with a comma in the account preferences.

1. Open Mail and choose Mail > Preferences.
2. Click Accounts.
3. Select an account.
4. Enter all your addresses in the email address field, separated by commas. For example, webmaster@example.org, sales@example.org, support@example.org.
5. Close the Preferences window and click Save in the message that appears.

Now you'll have a nice pull-down list when you compose email that allows you to select the address you want the email to appear to be from. You'll still need to choose the appropriate signature from your list -- but otherwise, perfect.

There you go: one step closer to inbox domination.