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The crime is deceptively simple and maddeningly effective.

Thieves, often at night, use string to lower glue-covered rodent traps or bottles coated with an adhesive down the chute of a sidewalk mailbox. This bait attaches to the envelopes inside, and the fish in this case — mail containing gift cards, money orders or checks, which can be altered with chemicals and cashed — are reeled out slowly.

This low-tech crime, which became common in the Bronx, is known as mail fishing.

Now it has met its match.

The United States Postal Service is replacing or retrofitting mailboxes in much of the northeast to eliminate the pull-down handle in favor of a slender mail slot with a singular goal: foiling thieves.