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.
Maps Dul Introduction: mail-abuse.org/dul/intro
Maps Dul is a bad idea: dt.e-technik.uni-dortmund.de/~ma/postfix/maps-dul-is-wrong
Moving beyond RBL's: theory.whirlycott.com/~phil/antispam/rbl-bad/rbl-bad
The digital imprimatur: fourmilab.ch/documents/digital-imprimatur