I have multiple e-mail aliases (all on the same server) that I need to send mail from. Since I’m running Mac OS X, my solution up until now has been to keep my primary address configured in Mail.app and my other addresses configured in Thunderbird.
Yes, I know that Thunderbird supports multiple identities, but I prefer Mail.app for everyday usage. I just kept hoping that Apple would update the program to support this capability. What I didn’t realize is that it already does.
It’s as simple as putting in a comma-separated list of addresses under “Email Address” in your account preference. This will cause a drop box to appear when you compose e-mails that allows you to select the “From” address used. Why Apple doesn’t make the capability more apparent is beyond me, but hey, at least it’s there!