Optus realised it had been placed on a global 'blacklist' of spammers commonly used by ISPs to block unsolicited bulk e-mail from reaching customers.
An Optus spokesperson told ZDNet Australia  it became aware of the problem last week and has been looking for a fix since. The company believes the spam list inclusion was a mistake.
According to a Telstra BigPond spokesperson, Optus was having difficulty staying off the blacklist and had approached Telstra for technical assistance in resolving the issue.
"Our spam experts are talking to their technical people to give them advice on what Optus can do to get off the blacklist.
"Telstra has the Spam National Operations Centre, which is a team of experts in spam, so we'll be lending them the resources to sort it all out," said the BigPond spokesperson. Due to the spam blacklist, BigPond has blocked e-mail from Optus.
Telstra was just being a good neighbour, the spokesperson added. Fixing the problem would also mean Optus customers will be able to send and receive e-mail to BigPond addressess.
Optus did not indicate when its e-mail service would return to normal.