(KMDL-FM) The United States Postal Service is warning customers to stop using the iconic blue mail collection boxes for mail drop-offs. Postal officials said customers who still mail checks should be especially cognizant of the check washing scam. What Is Check Washing? The USPS also suggests that if you do write checks, you use gel pens with black indelible ink. If you've been a victim of a check washing scam, there are some things you need to do as well.