Posties with parcels often knock on doors, which gives the dogs an opportunity.
Following a rise in canine attacks in 2015, the post office launched an app on postal workers' hand-held scanners that warns of dogs at certain houses.
Owners of repeat offenders are told to pick up post at a nearby post office.
If a loose dog plagues a postman, delivery to an entire neighbourhood can be suspended.
Carriers are also trained to use their satchels as barriers and carry pepper spray.
Some carry dog biscuits to distract aggressive and hungry mutts.
Dog attacks cost the postal service $2.8m in medical compensation and other expenses last year, up from $1.2m in 2011.
Half of the attacks in 2016 required medical treatment or time off from the mail route.
The postal service has asked customers to put dogs in a different room before opening the door.
But the danger is unlikely to disappear.
Another reason to hope that Amazon can make drone deliveries work—though those drones could be worth a nibble, too.