When you start typing a recipient's address or name in OS X Mail, the app seems to already know how to end what you've just started, even though the contact may not even be in your address book. Just because you don't see these contacts in your address book doesn't mean they're not saved. OS X Mail caches every email address you've ever sent a message to. You can make them more accessible by adding them to your address book.
Microsoft Outlook Email Autocomplete Options
Since OS X Mail obviously recognizes all these recipients, you’d think importing them would be easy. Good news: you’re right. You can harvest OS X Mail’s vast memory of all the people you’ve emailed to build your contact list in just a few steps.
To copy contact information from the OS X Mail Auto-Complete list to the Address Book:
