Step 1: in Mail, go to the Window menu and select “Previous Recipients.”
You’ll get a window like this one, but less blurry:
That’s a list of everyone you’ve sent emails to. Find the ones that are wrong, click on them, and use the “Remove From List” button to get rid of them. Problem solved.
Microsoft Entourage users: I haven’t forgotten you. You can indeed clear out an address from Entourage’s “Recently Emailed” list– but it’s an all-or-nothing affair. Apple’s Mail is better because it lets you pick and choose.
Here’s what it looks like in Entourage. Get there by going to the Entourage menu and choosing Preferences.
Of course you won’t see the helpful red box, but you’ll figure it out once you’re there.