If you send a message to, or reply to, someone who is not in your Address Book that email address is stored in Airmail. At some time you may want to find or take other actions with this address, such as deleting it, and Airmail makes it easy to do this.
Go to Menu > Window and select Address View which brings up the Address View screen below. To find a contact you can search for Name or Email Address.
Contacts that have the Apple Address Book icon beside them can not be deleted along with the addresses that have no icon, but those contacts which bear the Airmail icon can be deleted from this window.