To access your Business Email directly online:
Sign in to your Business Email account at http://mail.yourdomain.com/
Note: Be sure to replace "yourdomain.com" with your domain name. You'll be prompted to enter your Yahoo! ID and password if you haven't already.
Business Email Welcome Message.
Welcome Menu: Address List
To access your Business Email from your Yahoo! Small Business account:
From the Email Control Panel or Web Hosting Control Panel of your Small Business account, click the "Check Your Email" link or the "Check Email" button in the upper-right corner of the page.
Web Hosting Control Panel Check Your Email Button and Link.
Email Control Panel Check Your Email Button and Link.
Note: You will not be able to sign in to your mailbox if your domain is still pending.More about How do I sign in to my Yahoo! Business Email account?