They are going to be so pissed when they find out that the postal service can reject their letter because the sending or receiving party does not have a mailbox of an acceptable shape, size or made of acceptable material.

U.S. Postal Service STD-7B01

Postal Service, Code of Federal Regulations 39 CFR Part 111, Standard 7A, Mailboxes, City and Rural Curbside

See, I can look up random laws on Wikipedia too!