A council accidentally sent out contact details of more than 600 people instead of a "what's on" guide.
The email attachment from East Riding of Yorkshire Council contained names and addresses of the recipients.
The council quickly sent another email apologising and asking people to delete the original without reading the database.
Director of corporate affairs Darren Stevens said: "We are very sorry. It was a clerical error."
It is thought the email was sent to users of a children's centre in Hedon, near Hull.
Mr Stevens said instead of the planned guide the email included a database of details belonging to the people the email was sent to.
He said the email had not included any financial or health details and the council took data protection "seriously".
A review has begun into how the error occurred but human error could never be eliminated completely, Mr Stevens said.
"Think twice before any email is sent and make sure it has the correct attachment," was his rueful advice.