Incorrectly formatted email addresses don't get imported. If an address is imported, but doesn't exist, the system will soon detect this by tracking the bounce information and updating the contact to 'Bounced' status.
All bounced email addresses will still exist on the list, but won't be sent any emails going forward.
The system automatically handles unsubscribes for you. You shouldn't delete either bounced or unsubscribed contacts, as the system will need to identify which contacts not to send to based on the information associated with the contact.