Outlook has a feature that when you type an email address in the To section it remembers it and when you go to type another email address it will bring up similar choices in a drop-down, assuming they might be the one you want to use. But sometimes these are email addresses that have come back undeliverable as you had a typo in the address, or it's an old email address for someone who has since moved to another company and you don't want these old addresses to pop up. To get rid of the incorrect address when it pops up, just move down with the arrow key to the address you want to delete, when it is highlighted just press delete. It's as simple as that to get rid of it.
Do not click on it, just move down until the address is highlighted. If you highlight and click it or press enter it will use that address.
This also works the same in the CC and BCC sections of your email message