If you would like to use your Yahoo email address as the company address in Clear Books (Settings>Organisation>Details & Tax) to send invoices/bills/statements you will need to use the SMTP settings within Clear Books. In addition to this you will have to enable the less secure sign in option in your Yahoo account.
Navigate to the Home > Account menu.
Tick the check box to Use SMTP settings and add in the details.
Login to your Yahoo account.
Click on your profile name in the top right and hit the Account info link.
Click on the Account security tab and enable the Allow apps that use less secure sign-in setting.