As reported on TechCrunch and elsewhere, Yahoo's Chinese email service is no more. Warned all the way back in April, current users of the Chinese version of Yahoo! Mail were given the opportunity to transition their accounts to Alibaba's email service, Alimail.
As of January 1st, any attempt to mail a user at the yahoo.com.cn or yahoo.cn domains is rejected with a "550 relaying denied" error message.
If you run an email service that maintains a filter of dead ISPs or dead domains, I recommend adding yahoo.com.cn and yahoo.cn to your "dead domains" list or similar. There's no point allowing mail to be sent to those domains, as no mail will be successfully delivered.
There is nothing to indicate that users will automatically have the same username at Alimail that they had in Yahoo! Mail, so it likely is not safe for senders to just try to automatically update addresses in their email lists.