"This suggests that one reason there is so much spam from Linux could be that many companies that have implemented their own mail servers, and are using open-so

"This suggests that one reason there is so much spam from Linux could be that many companies that have implemented their own mail servers, and are using open-source software to keep costs down, have not realised that leaving port 25 open to the internet also leaves them open to abuse." So I closed my port 25. This fixed other problems as well. Since then all my customers stopped complaining via email.