Sending sms to owner of the yahoo account.
In my country Iran, everybody that enters to a bank account, a warning sms sends to cell phone of owner of the bank account to inform about entrance. This will help to distinguish and prevent illegal entrance by thieves or hackers. Likewise, Yahoo can send a warning sms to owner of Yahoo account.
If you can send activation/confirmation code to your clients round the world, then you can send warning sms to inform your clients about entrance.
With best regards.
Roozbeh Mohassel Akhlaghi (Mr.)