All spam comes from some where. If you have access to the servers logs you can identify exactly where. If you do not I can set you up so that you do. The phrase, “Knowing is half the battle”, rings true here.
Of the many subnets below 3 are from the US. I can get more into US business spam – which isn’t “opposed” by the spam-can act. Regardless – it is unsolicited and I feel no shame in sharing the information with the public.
The below networks are very broad. But I can assure you blocking mail from them will only help your situation. If you would like details on each network and why let me know.