The way this is handled in Finland that each isp has one outgoing SMTP-relay server that you have to use, you can't send the mail directly out. You can receive all the mail you want but the outgoing pipe has restrictions to prevent open/miss-configured servers, works great. (I have my own mail server with such arrangement on a static IP)
If you are a ISP I would suggest a similar arrangement to prevent all your customers sending spam
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The machines seem to be the database back-ends, and most of the large scale commercial billing/accounting/whatever applications like SAP want to have a unix backend. The users were probably all using windows workstations and windows apps that just communicate with the back-ends.