Schneier - Mail Fishing

Not email, paper mail:

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.

In response, the US Post Office is introducing a more secure mailbox:

The mail slots are only large enough for letters, meaning sending even small packages will require a trip to the post office. The opening is also equipped with a mechanism that grabs at a letter once inserted, making it difficult to retract.

The crime has become more common in the past few years.

from Schneier on Security https://www.schneier.com/blog/archives/2019/03/mail_fishing.html