Outlook 2010 suggests user with an input while filling out form or entering email address, subject, etc. All such information is extracted from Auto-Complete list. If at some point of time you need to empty Auto-Complete list, you can remove it with a single click.
To empty the auto-complete list, on File menu, click Options. It will bring up Outlook Options. From left sidebar, select Mail and from main window, under Send messages section, click Empty Auto-Complete List. A message box will pop-up confirming the action. Click Yes to proceed.
A click on Yes button will remove all the content from auto-complete list.