If a division of the department determines there is a deficiency in any state tax, it may send by first class mail or deliver a proposed assessment to the taxpayer. The proposed assessment must explain the basis for the proposed assessment and state that an assessment will be made unless the taxpayer protests the proposed assessment as provided in Section 12-60-450. The department shall make available forms which taxpayers may use to protest proposed assessments. The proposed assessment is effective if mailed to the taxpayer's last known address even if the taxpayer refuses or fails to take delivery, is deceased, or is under a legal disability, or, if a corporation, has terminated its existence. For a joint tax return, one proposed assessment may be mailed to both taxpayers unless the department has notice that the taxpayers have separate addresses in which event a duplicate original of the joint proposed assessment must be sent to each taxpayer at his last known address.