If you are using Mac Mail, when you start typing an email address in one of the field, it suggests email addresses that have been used previously. Sometimes this creates problems. For example, your friend gets a new email address and the old one no longer works. Yet, when you start typing their name, it suggests both and you are left trying to remember which address you are supposed to use.

There is an easy fix though. If you go to "Window -> Previous Recipients" in the menu bar, you'll get the list of email addresses that mail uses as suggestions. You can select addresses from this list and then click the "Remove From List" button to remove them from this list. Once removed, these addresses will no longer be offered as suggestions when you are typing in email addresses.