It was announced way back in August 2013 that Yahoo! Mail China would be shut down at the end of 2014. Today in 2015, any attempt to email any address at yahoo.com.cn or yahoo.cn results in bounce back (NDN) saying "550 relaying denied for <(yahoo email address)>." This happens whether or not the address was previously known to be valid. The door to this domain is truly closed for all. All you can do at this point is remove those addresses from any lists you might manage. You've missed your chance to ask those users (via email to their Yahoo! Mail China addresses) to provide you with an updated email address. If possible, consider contacting those subscribers through some other means and request that they provide you with an updated email address.
January 8, 2019 Update: I am reliably informed that the Yahoo China email domains are now up and running and are active again.