Microsoft Outlook maintains a nickname list that is used by both the automatic name checking and the automatic completion features. The nickname list is automatically generated as you use Outlook. If the nickname cache is corrupted, Outlook may not be able to identify recipients, may offer incorrect recipients when automatically completing the e-mail address, or may send the message to the wrong person.
This is a Outlook nickname and autocomplete issue.
Please refer to the Microsoft KB 28763 for more information.