USPS warning customers not to use drop boxes on Sundays and federal holidays --- here's why

The United States Postal Service is issuing a warning for customers who use their blue drop boxes.

CHICAGO - The United States Postal Service is issuing a warning for customers who use their blue drop boxes.

Officials are asking customers to avoid using those blue drop boxes on Sundays and federal holidays.

They say criminals are targeting drop-off boxes, and monitoring pick-up times.

USPS advises customers to sign up for informed delivery — which they can use to verify and track mail.

Officials are also advising customers to never drop off mail in mailboxes overnight.