Substantial compliance: procedural defects in tax proceedings do not invalidate actions if they satisfy statutory intents. Proceedings under the Act containing mistakes, defects or omissions shall not be invalid merely for those reasons if they are in substance and effect consistent with the intents, purposes and requirements of the Act or any existing law. Further, service of a notice, order or communication cannot subsequently be contested where the recipient has acted on it or where service was not challenged in earlier proceedings arising from that communication.