Scotland Yard detectives have seized thousands of items of scam mail aimed at people in the UK.
It is the first seizure of such post by the Metropolitan Police's economic and specialist crime command.
The letters were sent by professional fraudsters and encouraged people to invest in schemes like fake lotteries.
Police say an estimated £3.5bn is scammed from UK citizens each year, and about £2.4bn of this is believed to be as a result of mail scams.
The police operation - which was run in partnership with Royal Mail and international mail service provider Spring Global Mail - was part of a long-running police investigation into the organised criminal networks behind such scams.
Police expect to recover up to 5,000 items of mail sent by criminal networks around the world.
Also involved are Lancashire Police and council trading standards offices in Westminster, Kent, East Sussex and Hampshire.
The mail seized on Tuesday was taken from the Spring Global Mail centre, which is passing suspect letters on to police.
People from abroad are able to post their mail to the UK and Spring Global Mail will re-post it, making it look as though it was posted in UK, a legal practice which criminals are taking advantage of.
The Met say Spring Global Mail have done nothing wrong, and many other companies use similar services to make mail originating abroad appear as if it was posted in the UK.
The police also say they analysed some 3,000 scam letters and only one fifth were sent using this service, known as "local look".
As well as the mail seizure, five return addresses or "virtual offices" in London identified as part of the scam process are being shut down and searched for criminal evidence.
These addresses consist of privately rented mail boxes.
The fraudsters, who send letters from several overseas countries, try to persuade people to part with money on false promises of holidays or luxury items.
After replying to one of these so-called "tempter letters" a victim's name is put on what is called a "suckers list" and sold to criminals all over the world.
These lists comprise many vulnerable people, including the elderly, who will be bombarded with mail.
Some have ended up in desperate financial straits, and some with health problems. According to the Metropolitan Police, there have been at least five suicides involving postal scam victims.
Detective Superintendent Mark Ponting, of the Met, said once people reply to one letter they are targeted by hundreds more every week, and can find it hard not to reply to others.
"I've met quite a few now, several dozen individuals, who find it difficult if not often impossible, once they're hooked, to stop responding to the mailings," he said.
"So you can give them all the consumer education in the world and actually, once they're hooked, they're not going to stop."
Gary Simpson, international director at Royal Mail, said: "We don't want any of our postmen or women handling or delivering mail that causes harm or is criminal in intent.
"Royal Mail is working hard with the police to clamp down on this type of distressing activity."
Meanwhile, Conservative MP Caroline Nokes is putting a Bill through Parliament that would enable police and customs to be able to seize some of the scam mail which enters the country.
But although it could raise awareness of the issue, her Bill has little chance of becoming law because of a lack of parliamentary time.
Ms Nokes said some victims received more than 100 letters a day after being targeted by fraudsters, and that criminals also used the telephone and e-mails.
However, police were aware of how and where some scam mail entered the country but were unable to stop it, the MP said.
She said trading standards officers were keen for the police, customs officers, and the National Fraud Authority to intercept scam mail, and for Royal Mail to disclose the details of potential victims to the local trading standards service.
"I do not suggest for one moment that there should be a blanket power to intercept mail without a warrant, but such mail is easy to identify, the same victims are being targeted hundreds and hundreds of times over and it ought not to be impossible to introduce appropriate safeguards against breaches of human rights," she said.
Ms Nokes also accepted that current data protection and human rights legislation could prevent interceptions.