In Outlook 2010 there are a couple of way to work with this.
1. When you go to forward the message and the person's name automatically appears, simply hover the mouse over it and press DELETE. This will remove it from the cache. Now type in the address and the message will complete properly.
2. If you want to clear out all of your cache without trying to address them one at a time, then here are the steps:
- Open Outlook 2010
- Open File
- Click Options
- Click Mail
- Select "Empty Autocomplete List"
Microsoft's first recommendation is to clear your Outlook profile and create another one which is sure work but is a bit overkill for such a small issue.
I'm sure the process is similar with other versions of Outlook but YMMV.