The United States Postal Service has seen a rash of "mailbox fishing," the practice of thieves filching letters out of mailboxes, finding checks and then altering and cashing them.

"Where there's a will, there's a way, and crime won't stop," says U.S. Postal Inspector Glen McKechnie. "They all use the darkness. They'll go at night when the streets are empty and there is nobody out, and that's when we find that they are conducting their fishing expeditions."

One New Yorker confronted a man who was allegedly mail fishing by apparently slipping his belt coated with sticky stuff through the narrow mailbox slot. As she videoed him with her phone and yelling at him to stop what he was doing, the man continued and responded that he didn't care that he was on camera before driving off on a moped.

The New York City Police Department has plastered signs on some blue postal boxes warning people about mailbox fishing.

photo shows close up view of signage on NYPD vehicle

The New York City Police Department has plastered signs on some blue postal boxes warning people about mailbox fishing. (iStock)

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"Sending a check or money in the mail? Beware of Mailbox Fishing!" says the signs, explaining that people should "drop mail that contain checks directly at the post office."

Earlier this year, a ring believed responsible for stealing more than $100,000 worth of checks was busted; another suspect in September was caught with $84,000 worth of checks. The campaign of New Hampshire U.S. Sen. Jeanne Shaheen had two checks worth $21,583 fished out of a mailbox; and in August, Mattapoisett, Massachusetts, police arrested two teens for allegedly stealing and altering checks worth $250,000 from mailboxes.

Isabel Vincent said two checks she once mailed were fished out of a box, altered and cashed for more than $1,500.

"Are we going to see a world where mailboxes are gone? I mean, that would be very sad," she says.

Vincent's checks were doctored, with someone washing out the payee and writing another name then cashing the checks. Even though her bank eventually reimbursed her for the loss, she says that she now takes care to only mail checks at the post office or pay electronically.

Mail dropbox in Philadelphia being targeted by thieves

Authorities say a mail dropbox outside a Pennsylvania post office was struck dozens of times by thieves searching for money and checks. (FOX 29 Philadelphia)

"Don't pop a check in the box. Go personally to a post office," she advises. "It's a bit of extra time, and you don't have the convenience of doing it in front of where you work, which is where I used to do it, but at least you know you have a modicum of trust that your check will make it to its destination."

McKechnie advices people to post their mail near the box pickup time so that letters don't sit in a box overnight, when most of the fishing occurs.

'Every blue collection box on the street corner has a time on it when the box is swept. We're asking customers to put the mail in the box before the collection time. If the mail sits in that box overnight, it could be stolen."

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The Postal Service inspectors are cracking down on the practice and have taken steps to make mailbox fishing more difficult. Thousands of boxes have been retrofitted to remove the old swinging door to narrow the mail slot. While that has caused a decrease in thefts, they still continue. McKechnie says if you think that you have been a victim, police should be contacted.

"We work with the precincts to gather that information," he says. "We're the Postal Inspection Service. We have to instill trust in the mail. Those boxes are safe, and we want to get the message out to mail before that last collection time."

Mailman delivering mail from truck

A mailman reaches out of his truck to deliver mail, Oct. 2, 2021, in Fort Lauderdale, Florida. (iStock)

The fishing has expanded to armed robbery. Last month, a Chicago mail carrier was robbed at gunpoint for his mail box key, giving thieves access to the boxes that hold bulk mail. In Santa Monica, California, thieves bashed in mail boxes to steal their contents.

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"When you mail a check, look at your statements. Make sure that the check was deposited or the right person deposited the check," said McKechnie, who added that people who are victimized should report any thefts to the police and to the Postal Inspection Service.

Jennifer Johnson contributed to this report.