Network address translation can be used intercept all port 25 (SMTP) traffic and direct it to a mail server that enforces rate limiting and egress spam filtering. This is commonly done in hotels, but it can cause e-mail privacy problems, as well making it impossible to use STARTTLS and SMTP-AUTH if the port 587 submission port isn't used.
Machines that suddenly start sending lots of e-mail may well have become zombie computers. By limiting the rate that e-mail can be sent around what is typical for the computer in question, legitimate e-mail can still be sent, but large spam runs can be slowed down until manual investigation can be done.
