I had the same problem. I’m using the incoming mail server and tried adding qq.com to the blacklist, but it didn’t work, I think because they have apparently hacked a bunch of servers, so the mail isn’t coming from qq.com, but instead the hacked server, so the blacklist doesn’t work.
I ended up just disabling incoming email altogether. (Probably not a solution for you).
And it does seem that discourse is sending a “Your email didn’t work” message to all of those.