77 Pa. Stat. § 1506
All notices and copies to which any party shall be entitled under the provisions of this article shall be served by mail, or in such manner as the board shall direct. For the purposes of this article any notice or copy shall be deemed served on the date when mailed, properly stamped and addressed, and shall be presumed to have reached the party to be served; but any party may show by competent evidence that any notice or copy was not received, or that there was an unusual or unreasonable delay in its transmission through the mails. In any such case proper allowance shall be made for the party's failure within the prescribed time to assert any right given him by this act.
The department, the secretary of the board, and every referee shall keep a careful record of the date of mailing every notice and copy required by this act to be served on the parties in interest.
77 P.S. § 1506