39 C.F.R. § 491.3
No document purporting to garnish employee wages shall be deemed sufficient unless it can be determined from the face of the document that it is legal process in the nature of garnishment; that it is issued by a court of competent jurisdiction or an authorized official pursuant to an order of such a court or pursuant to federal, state or local law, evidenced by a signature of the issuing person; and that it contains the name of the garnished party, with his or her social security number, orders the employing agency to withhold from pay a specific amount of money, specifically describes the judgment of debt or administrative action complete with statutory citation and contains specific advice as to where to send the funds as they are periodically withheld including the complete Zip Code (Zip + 4). When there is a suggestion that the employee is under the jurisdiction of a bankruptcy proceeding, the creditor must provide documentary evidence to prove that his legal process is not in violation of the bankruptcy court's jurisdiction before the creditor's garnishment may be processed. Documents deficient in any of these respects may be returned to the issuing court or authorized official inscribed "Insufficient as to legal form."
39 C.F.R. § 491.3