Most ISP’s and network providers have specific email addresses that you can report abuse to.
These addresses can be looked up using a whois tool.
Recently, I’ve noticed a number of those addresses that just don’t work the way they are supposed to.
A while ago I added a new anti-spam technique to my mail server setup called “No Listing”.
No listing basically means putting a high priority MX record in the DNS that always rejects mail server connections..
It seems, however, that the qmail mail server can’t cope with that. It’s not RFC compliant.