Maps Dul is a list of Dial-Up IPs (which also includes cable and ADSL clients) that some SMTP administrators are using to block legitimate E-mails from reaching them.

This is my case, which means I can't send emails to some big-company domains and I have to use other email systems to do it. I can't use my "ISP" relay, because I can't afford to wait four days for an email to reach the destination.