The commissioner of the department of employment security or his authorized representative shall make an initial determination on all questions relating to the liability of employing units or employers or both, including the amount of contributions, the rate at which contributions shall be paid and successorship. A copy of the initial determination shall be sent by certified mail, return receipt requested, or first class mail, whichever the commissioner determines to be most appropriate, to the last address, according to the records of the department, of each employing unit or employer affected thereby.