To begin, type the email address of the person you wish to delete, stop typing as soon as you see their name in the drop down list.. In this example we are using firstname.lastname@example.org (and we stop at s)
Next, use the up and down arrow keys to highlight the entry you wish to delete.
Finally, hit the delete key to remove the entry from the list.
The email address will eventually return to the drop down list if you type it again, so this is a good way to clear out old email addresses.