OzEmail customers have complained of being "blacklisted" by an anti-spam organisation that has prevented many e-mails from reaching their intended recipients.
One customer who contacted PC World said OzEmail admitted more than one hundred subscribers had complained to the ISP about the same problem. The customer said his e-mails had been rejected from large enterprises, including those in the banking and retail sectors.
The error message the customer received was "550 Mail from 188.8.131.52 refused by blackhole site rbl.maps.vix.com". The URL it referred to is for the Mail Abuse Prevention System (MAPS), which posts spammers to the Realtime Blackhole List (RBL).
Black listing is usually associated with spamming.
"The MAPS RBL is a system for creating intentional network outages for the purpose of limiting the transport of known-to-be-unwanted mass e-mail," the US-based Web site explains.
"The MAPS RBL is a subscription system, such that no one is ever denied connectivity to a non-RBL-subscriber. If your network seems to have been 'blackholed' by us, be aware that the places you cannot reach have deliberately chosen not to exchange traffic with you. We are not the network's police force, but rather, a method to identify likely spam origin."
However, an OzEmail spokesman denied it had done anything wrong.
"It's probably an overstatement," said OzEmail's Michael Ward. "It's not our servers."
Ward explained the black listing had most likely occurred from one of OzEmail's clients who has a mail server connected to the network. He would not name the responsible client.