The personal details of 817 E.On customers have been disclosed in error.
E.On said it was trying to find out how it happened and was in the process of contacting all of those affected. It has apologised for the mistake.
The energy company said it had managed to speak to about half of those customers affected.
A customer from Coventry said she was sent bank account details, mobile phone numbers and the names and addresses of two people in Bradford and Nottingham.
The woman, who did not want to be named, said: "When I called E.On they said there had been a mix-up.
"All they knew was people's bank details had been sent to the wrong people.
"What I don't know is, who has got my details?"
An E.On spokesman confirmed 817 customers had been affected nationally.
He said: "We use a third party mailing service to send the letters out, but this error is our problem and we apologise.
"It seems that customers received letters relating to their own address, and two additional letters for other addresses."