Sometimes when you start to type inn an email address Outlook will try to predict what your are going to type in, and wants to help you by Auto Complete it for you.
If you want to remove them, simply follow these steps to remove the history addresses.
Open a new mail.
Start to type in the address you want to send to.
Outlook will at some point come up with a suggested address.
When the address is showing up in a drop down menu, simply hit the cross to the right of the address to delete it from the stored address history list.