If I were a spammer, I don’t want to use the SMTP servers of Comcast or Surewest, even if I am a customer of one of these ISPs. The reason is simple. Most commercially run ISPs detect at least outgoing spam messages. And, furthermore, my ISP can link all those spam messages back to me (since I have to log in to the network to get connectivity to begin with).
As a result, spammers look for SMTP servers that generously provide relay service regardless of the origin of a message. As an administrator, you need to make sure your SMTP servers are not generous.