Today I stumbled upon a blog post in Hawk Wing’s blog which was written in 2005 but is still up-to-date: even with the latest version of Mail.app included in Mac OS X Leopard, you are able to use multiple e-mail addresses for one POP or IMAP account.
Within the mail account settings you will have to provide a comma-separated list of your e-mail addresses (e.g.
firstname.lastname@example.org, email@example.com, firstname.lastname@example.org) and then you can choose the address you want to use as the sender’s address from a pop-down menu in the “New Message” window. Nice 😉
More tips for Mail.app can also be found on Apple’s support pages.