The USPS Domestic Mail Manual states that “no part of a mail receptacle may be used to deliver any matter not bearing postage, including items of matter placed upon, supported by, attached to, hung from, or inserted into a mail receptacle.” In other words, the mailbox may not be used for anything other than for pieces of mail with postage attached. The USPS Domestic Mail Manual goes on to states that it is a federal offense and there is a significant penalty for violating this law which could include jail time.
At least he didn’t send “junk” mail...
So, when putting dog doo Ina mailbox, attach a stamp.