"If you have several email accounts, and use Apple Mail as your main application, you can set up multiple account names," De Prez Allen Laudenslager reminds us.
The Apple instructions are right here for you.
"What this means is that when you send a new email, you can select from any of your accounts and the mail will be sent with that as the visible address and not from your 'primary' address.
"For example, suppose you had an account named firstname.lastname@example.org as your primary but also had an email@example.com. With this system from Apple mail, you can send your message from either your .mac or .gmail account directly from your mail application!
"Cool, huh?" Yes, Allen, and thanks for keeping us informed and inspired!