A letter of protest is written in a highly formal manner, intended to be both courteous and critical at the same time. For example, in order to express exceptionally strong anger towards a foreign government's policy, a letter of protest would say, "We condemn this action in the strongest possible terms."
In business, a letter of protest is a communication in a newspaper intended to warn potential lenders of unpaid debts by a debtor, such as a company or sole proprietor. Getting into the "protest list" usually hurts the debtor's credit rating and leads to difficulties getting unsecured loans. As such, it is a cheaper way to punish non-payers than to sue the debtor. Furthermore, publication in a private newspaper allows earlier warning as compared to the official journal, which publishes only final verdicts from courts.