I had a few holes in my anti-China filtering which have now been rectified.
The iptables rules were not in the right order, and my list of netblocks was out of date.
In fact, today so far, not a single spamming connection has arrived to my mail server! The reject log is completely empty. [Cue sound of crickets.]
The ironic thing here is that I am interested in what "Wintop" are selling: PCB manufacturing. But I'm not going to give business to spammers.
Moreover, these people have an idiotic website, where you don't find anything useful. Just claims that they can make nice PCB's, documented with pictures of customer boards (pleasure to see, but so what?), and links to "contact us", "we give you good price".
You know what? I ... don't ... want ... to ... contact ... you!
I do not want to chat, let alone with people who probably use Google translate between Chinese and English! I want to see, on your main page:
These maroons don't have any of this, or not in any place where it is obvious (the main landing page). No wonder they have to resort to desperate spamming techniques to drum up business.
So this just goes to show you: spamming does not necessarily work even when it reaches potential customers who are interested.