Just because you don’t see these contacts in your address book doesn’t mean they aren’t stored.
OS X mail caches every email address to which you’ve ever sent a message.
You might want to make them more accessible by adding them to your address book.
Screen shot courtesy of Coyote Moon, Inc.
Given that OS X Mail obviously recognizes all these recipients, you might think importing them should be easy.
Good news: You’re right.
