Notices, directions, orders, and documents required by this Act, or by any regulation made under this Act, to be served or sent may, unless otherwise expressly provided, be served and sent by post, and, until the contrary is proved, shall be deemed to have been served and received respectively at the time when the letter containing the same would be delivered in the ordinary course of post; and in proving such service or sending it shall be sufficient to prove that the letter containing the notice, direction, order, or document was prepaid, and properly addressed, and put into the post.