Energy supplier E.On has apologised for an "error" which saw hundreds of customers' email addresses included on requests for their meter readings.
The automatic messages should have been addressed to each individual only, but had another 497 recipients listed.
E.On said the messages were the result of a "system error" and were spotted "within minutes" of being sent.
In support forums on E.On's website, some customers warned they may refer the firm to the UK's data watchdog.
In a statement, E.On said it had "apologised for an error which happened when an email was sent to a limited group of customers requesting meter readings".
The company said it was talking directly to customers who had raised concerns about the sharing of their details.
'Duty of care'
E.On added that no account information or financial details were included in the emails.
"An internal investigation is under way, and the appropriate authorities will be notified where required," said the energy supplier.
Tony Pepper, founder of cyber-security firm Egress Technologies, said it was not surprising that E.On customers were worried about personal email addresses being shared.
"E.On has a duty of care to protect such information from any risk of falling into the wrong hands, so it will be interesting to see what they intend to do to resolve the slip-up," he said.
Although it was unlikely that E.On customers would suffer as a result of the breach, Egress had seen other similar blunders that had put customers and their data at risk.
"This is a simple but sometimes devastating mistake to make," he said.