Fortunately, this problem is very easy to solve, though, to be honest, I don't have a ".mac" account, so I had to ask someone who did -- Josh St. Germain -- and he was kind enough to track down your likely problem and offer up a solution that's not too hard to implement.

Josh, take it away!

Okay, Dave, first off, the person who emailed in the question needs to click the "Edit Message" option. Next, at the top-left of the screen, click the "Mail" dropdown. Click "Preferences" and a window will pop up, like this:

In this window, click on the "Composing" tab to make sure you're in the same place I am here, then, under the "Addressing" section, there will be a dropbox with "Send New Mail From:" next to it. You should be able to successfully select your account in the drop menu.

It looks like this:

After you have selected your mailbox, you can click the red "X" in the top-left of the window and you are ready to re-send your email. Fixed!

Happy Messaging!