Speaking as one of the targets
Actually, I suspect this may have been the guy I've been harassed by for years. I'm still not sure how he got my email address in the first place, but I've always suspected that his real scam was charging for advertising. He probably wasn't a yakuza himself, but kind of an affiliate providing advertising services for the yakuza's prostitutes and loan shark operations. Some days he'd send 50 spams, often repeating the same "ads" over and over again.
Assuming it's the same guy, then his websites have been becoming increasingly mobile over the last year, though he's switched from using dynamic DNS services to using registered domains. His original scam was apparently rather too complex, especially when one of his international DNS suppliers switched his DNS records to point at the website of the Japanese National Police Agency. I rather hope that startled a few of his wannabe customers--and I don't think he ever used that DNS again. There was also a Danish guy who simply gave me a special account that I could use to cancel the spammer's DNS records. Another service the spammer quickly abandoned--but he kept right on searching for new ones.
I'd even like to consider that i might have contributed to his final downfall, though I obviously don't want to make a public statement about the new method I started a few weeks ago. Might be worth using again. I'll just drop the hint that Japanese politicians are frequently tainted by associations with yakuza, and they try to avoid them no matter how much they like the yakuza's political support.
However, I actually think it most likely that the police set him up with a simple sting operation. The spammer seemed to be an idiot anyway.
No spam has arrived in a while. I certainly don't miss it.