One couple was conned out of £600,000 over a 10-year period, trading standards officials said.
They were conned out of £300,000 in a "land banking" scam, buying expensive pockets of land with false promises of development.
A similar amount was lost on shares bought from abroad, sold as investments but in fact worthless.
The county council said such mail scams targeted the elderly and lonely and often involved bogus lottery wins, clairvoyants and inheritance frauds.
The couple's son, who asked only to be identified as Michael, said he was still dealing with bailiffs over the debts.
"My mother has sufficient pension income for her immediate needs so it hasn't left us completely cleaned out of money" he said.
"I'm afraid we've ended up in a situation where, out of my father's lifelong habit of saving and investing, the criminals have had the greater part of it and the rest of us are left with relatively little."
Trading standards officers highlighted the case as part of Scams Awareness Month.
They said the average age of a mail scam victim in Derbyshire was 77, and 90% have recently been widowed or live alone. Bereavement, isolation and loneliness were common factors, they added.
Manager Steve Allen said: "We think they started with mail scams, maybe smaller amounts of money.
"Over time they started to get fraudulent telephone calls, signed up to a land banking scam and the information was shared. "
Elderly people and other vulnerable groups in Derbyshire have been warned to be aware of mail scams.