Gmail now automatically adds people that you corresponded with to 'Suggested Contacts'. You can move people you correspond with regularly to 'My Contacts'. Gmail's address auto-complete feature works for all of your contacts, whether they're in 'My Contacts' or 'Suggested Contacts'.
Furthermore, to ensure that only the right contacts are synced with your mobile device in the future, Gmail is moving previously auto-added contacts in 'My Contacts' back into 'Suggested Contacts'. Only contacts that you've moved to 'My Contacts', edited, imported or added to a group will remain in 'My Contacts' for syncing.
For more information: