Putting an envelope in a U.S. Postal Service mailbox should be as secure as putting an infant in its mother’s arms.

Maybe it once was, but not anymore.

In early June, lawyer Diane Hlywiak put an envelope containing a check for $1,200 into the big blue mailbox at Calumet and Dobson Streets in East Falls. It was her quarterly tax payment to the state Department of Revenue.

A few days later, while checking her Wells Fargo account online, Hlywiak was shocked to see that a check for $4,200 had cleared on June 10. She viewed the check online and saw that her $1,200 payment had been altered and made payable to someone named Karanja Edwin Lake.

She immediately filed reports with Wells Fargo, police, and postal authorities. She then posted her experience on the East Falls Nextdoor neighborhood website. It turned out the same thing happened to two neighbors, she tells me.

“One neighbor’s check was altered for $5,000 and the other was altered for $7,500,” she says — and one of them also was made out to Karanja Edwin Lake.

“So, someone either broke into three different mailboxes in the neighborhood,” says Hlywiak, 42, “or this is an inside job.”

East Falls neighbor Dani Slon put on a sleuthing hat and found a Karanja Edwin Lake on Google. You could do the same. Are postal authorities doing it?

Apparently not. I called the U.S. Postal Inspection Service to ask what it’s learned.

“I have no specifics as to what has been done,” says Rachel, who is not allowed to use her last name.

East Falls resident Anita Alvare, 66, says she mailed a $500 check to a charity from a mailbox about four blocks away at Midvale Avenue and McMichael Street, and soon began receiving overdraft notices from TD Bank. That’s when she learned her check had been altered to $6,500, and made out to a James Galaby.

“I write in cursive, the check was in block print,” says Alvare, who owns a marketing agency. She promptly notified TD Bank and police.

This type of theft has been going on for a couple of years, Alvare says she was told by Detective Christian Chaves, who referred my questions to the Police Department’s public affairs unit, which says it has no answers about mailbox thefts.

About a month ago, to pay a tree service company, medical researcher Devida Long put a check for $1,800 in the mailbox at Midvale and McMichael. When she was billed by the tree service, she found that the check had been deposited in an account at Bryn Mawr Trust even though it had not been endorsed. Long notified her bank, as well as police and postal authorities. Her check “was still addressed to the tree service,” she says. Her bank, Bank of America, needs to take that up with Bryn Mawr Trust.

It’s disturbing to learn that mailboxes are not secure, and the residents of East Falls are not alone. Authorities say pilfering USPS mailboxes happens across the nation, usually using a technique called fishing, which works just like how it sounds.

In discussing mail problems, East Falls Community Council president Bill Epstein tells me many East Falls mailboxes have been replaced with a type that do not open wide enough to be “fished.” The East Falls post office, he says, has been the subject of complaints for a long time.

“I have lived in East Falls for 25 years, and the East Falls post office has been a problem for as long as I can remember,” says Julianne Sullivan, 52.

Alvare wants the East Falls post office investigated.

I would make it post haste.