Almost 200,000 potential victims who appear on so-called "suckers lists" used by scammers have been identified by Trading Standards teams in a crackdown on fraudulent mail.
National Trading Standards (NTS) said the average age of those on the lists was 74, showing that criminals tended to prey on older and often vulnerable people.
The 10,843 identified victims lost an average of £1,184 each, or just over £13 million in total.
However work by the National Trading Standards Scams Team (NTSST) had saved consumers more than £5 million over the past three years, its figures showed.
Since its inception, the team had directly stopped at least £2.7 million from making its way in to the hands of criminals and had supported local trading standards teams to make savings of a further £2.3 million.
The NTSST is releasing the figures to highlight the scale of the issue during Scams Awareness Month and to urge people to be vigilant and look out for relatives or neighbours who may be vulnerable to mass marketing scams.
NTS chairman Lord Toby Harris said: "To have saved consumers more than £5 million in three years is a great achievement and shows the powerful effect the National Trading Standards Scams Team is having. However, we know our work is not done.
"Criminal scammers are targeting some of the most vulnerable people in society, ripping them off in many cases for thousands of pounds.
"We are going to continue in our fight to protect consumers and we urge you to help us by reporting suspected cases of postal fraud to the Royal Mail."
NTS and the Royal Mail are continuing with a joint initiative to train postal staff to recognise tactics used by mail scammers and to identify potentially vulnerable households.
Training sessions took place at Royal Mail delivery offices across the UK this week and more than 2,000 staff have been trained to date, resulting in more than 70 households being helped to avoid falling prey to mail scammers, NTS said.
Louise Baxter, team leader of the NTSST, said: "We really need the public to help us with this by being vigilant about mass marketing scams themselves but also looking out for relatives or neighbours, particularly those who are elderly or vulnerable.
"We often find victims who have lost hundreds of thousands over several years. The impact on individuals and consumers is devastating.
"I would urge anyone who receives potentially fraudulent mail or who knows someone who might be receiving it to report it to the Royal Mail helpline on 03456 113413 or to email firstname.lastname@example.org"
Citizens Advice figures show that two in five of all postal scams are lotteries or prize draws, inviting people to claim a prize for a competition they have not entered.