Perth ISP iiNet had some of its email servers placed on one of the world's largest spam blacklists for the second time in a week after a gaffe by a "junior" staff member.
Spamhaus, which tracks the world's largest spam operations, last week blacklisted an IP address range used by iiNet.
iiNet operations centre manager Richard Staniforth said the problem was isolated to a customer's dedicated server, which was located in the same IP address range as iiNet's mail servers.
The dedicated server was removed to prevent further difficulties for iiNet mail users.
The problem appeared resolved. But it resurfaced, Staniforth said, when a "junior" call centre operator accidentally derailed the process after sending Spamhaus a blank delisting email.
"There was basically a blank email sent to Spamhaus that said 'please remove' [us from the blacklist]," Staniforth said.
"And so they went 'Oh, but you guys aren't doing anything about it, let's list you again'."
News of the blacklisting broke on broadband forum Whirlpool when iiNet customers began alerting the ISP to a problem of emails not being delivered.
Other email service providers were blocking delivery of the emails based on information being supplied to them by Spamhaus.
Spamhaus provides a free list to email service providers that they can use to help block spam.