Microsoft Outlook creates a nickname list that is for name checking and auto completion. When you use Outlook it automatically creates the nickname list. The nickname list can become corrupted and when it does Outlook may not identify recipients, may suggest the wrong recipients with automatic completion or it might just send the message to the wrong person!
To fix this, close Outlook and then open up Explorer. Select Tools | Folder Options and click on the View tab. Select Advanced View and check the boxes next to Show Hidden Files and Folders. Next, open up the Search applet (don't forget to say hi to
Search Puppy!) and search for *.NK2 files. There will be a NK2 file for each Outlook profile on the computer and it will be named profilename.NK2. Rename the file to profilename.bak or delete the file. When you open up Outlook, it will create a new NK2 file and start a new nickname list.