The Google Apps Directory is automatically enabled on your account, but you can make sure with these steps:
1. Open Airmail App
2. Open Airmail Preferences (⌘,)
3. Click on the settings icon next to the MS Exchange account
4. Click on the Composer tab
5. Make sure the Directory Search is ticked
6. Click on Save
Using Google Apps Directory
Once the Directory Search is activated, Airmail app will automatically search the Directory.
1. Open the 'New Message' window.
2. Start typing the name of the recipient in the 'To:' field.
3. You will notice the loading icon on the right. That indicates that 'Directory' is being searched. 4. Once a result is yielded, it will appear in the list.
5. Click on the appropriate email address.