Outlook remembers and stores the e-mail addresses you type in the From, To, Cc, and Bcc boxes. When you begin entering a previously typed e-mail address, Outlook automatically recognizes it and completes the text for you. To accept a suggestion, all you have to do is press the ENTER key.
For e-mail addresses with the same first three characters, Outlook provides all applicable suggestions in a drop-down box. Use the UP ARROW key and the DOWN ARROW key to select the e-mail address you want, and then press ENTER. If stored addresses exceed seven, a scroll bar will appear.
Note: To delete stored e-mail addresses from the drop-down box, simply use the arrow keys to select the e-mail address you want to delete, and then press DELETE.
Disable autocomplete and clear cache in Outlook 2010 and 2013
Click the File menu and select Options.
In the Outlook Options window click the Mail tab.
Scroll down to Send messages. Uncheck the Use Auto-Complete List to suggest names when typing in the To, Cc, and Bcc lines box.
Disable autocomplete in Outlook 2007
On the Tools menu, click Options.
Click E-mail Options, and then click Advanced E-mail Options.
Under When sending a message, clear the Suggest names while completing To, Cc, and Bcc fields check box