Whoever, being a postmaster, affixes his signature to the approval of any bond of a bidder, or to the certificate of sufficiency of sureties in any contract, before the said bond or contract is signed by the bidder or contractor and his sureties, or knowingly, or without the exercise of due diligence, approves any bond of a bidder with insufficient sureties, or knowingly makes any false or fraudulent certificate, shall be fined under this title or imprisoned not more than one year, or both; and shall be dismissed from office and disqualified from holding the office of postmaster.