To access your Business Email directly online:
Sign in to your 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.
Business Email Welcome Message Pull-Down.
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?