In every proceeding for the recovery of any postage or other sum alleged to be due under this Act in respect of a postal article,--
         (a) the production of the postal article, having thereon the official mark of the Post Office denoting that the article has been refused, or that the addressee is dead or cannot be found, shall be prima facie evidence of the fact so denoted, and
         (b) the person from whom the postal article purports to have come, shall, until the contrary is proved, be deemed to be the sender thereof.