In a post to the aaNet forums just after midday, the ISP's chief John Lane said that services had been restored "but the mail server is processing the backlog of mail, which is very large and will take some hours".
In an subsequent email interview with iTnews, Lane said that "Yesterday's failure was a hard drive failure, which cascaded into some form of RAID array failure, with the consequent requirement to re-build the data sets."
EFTel's head of customer service, Ryan Bunter, separately apologised to users for the outage.
Bunter is the recently appointed direct report for aaNet's general manager, Jason Mikronis.
"The impact on members as a result of our mail issues is extremely regretable and something that we are genuinely sorry for," he said.
"It is of course reasonable to expect a reliable email service from your ISP and our mail service simply has not met expectations."
Lane said the ISP had existing plans to deploy a new email cluster to support its growth.
"We're confident it will put paid to these [email] issues permanently," Lane said.
"We have been planning to replace the aaNet cluster for some time, and one event after another has intervened to prevent us from completing the project."