I have multiple addresses that resolve to the same email account. Since I’m subscribed to most NC State lists and in the address books for most people with my @ncsu.edu email address, I need to use that when posting to those lists, or sending mail on campus.
Externally though, I want to use my @extension.org address.
Separating the addresses with commas in the Account Preferences for Mail.app lets me click the Account: button:
and pick from multiple addresses:
Hooray for the lazyweb!