I get from basically all incoming providers the error code: 554 5.7.1 Service unavailable; Client host [***] blocked using dun.dnsrbl.net (http://dun.dnsrbl.net/) If i disable dun.dnsrbl.net everything works fine. Does anybody know of any current errors?
Seems like they forgot to renew their domain name and now every query tells you the sender is blacklisted. So (temporarily) disable it.
You might also consider switching to weighted rbls so it doesn't block mail the next time it happens. Eg. Use postscreen at mta level, and scored rbl results is spam scanner.