Kmart breached spam laws more than 200,000 times despite multiple warnings from the media watchdog.
The Australian retailer has paid a $1.3 million fine for having sent a stack of marketing emails to people who had already unsubscribed.
An Australian Communications and Media Authority investigation found technology, system and procedural failures caused the massive error.
But before it had even opened the investigation, ACMA says it alerted Kmart a number of times it might have issues with its marketing compliance.
It breached the laws between July 2022 and May 2023.
"Kmart was given more than enough notice it may have a compliance issue and it should have done more to address its problems before we had to step in and investigate," ACMA chair Nerida O'Loughlin said.
"Kmart's case is particularly concerning as it went on for such a significant period."
Ms O'Loughlin said Australians were sick of big brands disregarding their wishes and invading their privacy.
"When a customer decides to opt out of a marketing mailing list, businesses are obliged to fulfil that request … the rules have been in place for nearly 20 years and there is simply no excuse," she said.
"Any business that conducts e-marketing should be actively and regularly reviewing its processes to ensure it is complying with the rules."
A spokeswoman for Kmart said the company regretted that customers received emails when they had opted out.
"These issues should not have occurred and we are actively working to strengthen our systems."
