Security – How to Check the Authenticity of a Domain

According to Yahoo's security page (emphasis mine):

1. Unofficial "From" address: Look out for a sender's email address that is similar to, but not the same as, a company's official email address. Fraudsters often sign up for free email accounts with company names in them (such as "ysmallbusiness@yahoo.com"). These email addresses are meant to fool you. Official email from Yahoo! always comes from an "@yahoo-inc.com" email address.

Note: Fraudsters can forge the "From" address to look like a legitimate corporate address (like "@yahoo-inc.com"). Because of this, the "From" address is just one factor to consider when deciding if an email is trustworthy.

I have personally received emails from yahoo-account-services-us@cc.yahoo-inc.com when it was necessary for them to contact me about my account. As the above note says, though, someone could still spoof this address like any other, so keep an eye out. You can probably contact Yahoo's corporate office, via this form to verify that an email is legit.