Outlook has a featured called "automatic completion" for any email address typed into the To or CC fields. This feature does not pull from the contacts or address book, rather, it adds names to the auto complete list as you type them. The problem comes in when you add a bad address, and then each time you start to type in the address, the incorrect one appears in the list.
To fix this problem, (drum roll, please...):
1. On the To: line, start typing the address that shows up incorrectly.
2. Hit the Down arrow key to select the 'bad' address.
3. Hit the Delete key.
Shockingly simple.
On the other hand, what if your auto-complete has never worked and you don't know what I was just talking about?
Here's how to activate it...
1. Click Tools, Options, Email Options
2. Click "Advanced Email Options"
3. Select the check box beside "Suggest names while completing
To, CC, BCC fields"