Even though many people use the free Yahoo account for business Yahoo does not want you to. They have added an indemnify clause to their terms to protect them against any problems you have. If due to Yahoo mail problems you lose business Yahoo has no responsibility.
Yahoo scans accounts and can lock ones that it finds are being used for spam. If there is no age the account is treated as belonging to an underage person and is deleted.
If you are running a business professionally you should get a business account. I would not trust a business that is using free email. That is what scammers use. You can get a business website and email for about $10 a month from various companies.
According to the terms of service it actually seems that Yahoo does not allow an account to be used for non personal use.
“Software License. Subject to your continuing compliance with these Terms, Oath grants you a personal, royalty-free, non-transferable, non-assignable, revocable, and non-exclusive license to use the software and APIs provided to you by Oath as part of the Services.”