Deemed service of notice prevents later objections where assessee appeared or cooperated, unless objection raised before assessment completion. Where an assessee appears in or cooperates with a proceeding or inquiry relating to assessment or reassessment, any notice required under the Act is deemed to be duly served in time and in accordance with the Act, and the assessee is precluded from later objecting that the notice was not served, not served in time, or served improperly, provided the objection was not raised before completion of that assessment or reassessment.