1. Except as otherwise provided in this section and section 143.721, a notice of deficiency shall be mailed to the taxpayer within three years after the return was filed. No deficiency shall be assessed or collected with respect to the year for which the return was filed unless the notice is mailed within the three-year period or the period otherwise fixed.

2. If a taxpayer other than a corporation omits from his return an amount of income that is properly includable in his Missouri adjusted gross income and which is in excess of twenty-five percent of the amount of the Missouri adjusted gross income stated in his return, a notice of deficiency may be mailed to the taxpayer within six years after the return was filed. If a taxpayer corporation omits from its return an amount of income that is properly includable in its gross income from all sources within this state which is in excess of twenty-five percent of the amount of gross income stated in its return from all sources within this state, a notice of deficiency may be mailed to the taxpayer within six years after the return was filed. For purposes of this subsection, in determining the amount omitted there shall not be taken into account any amount which is omitted in the return if such amount is disclosed in the return, or in a statement attached to the return, in a manner adequate to apprise the director of revenue of the nature and amount of such item.

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3. If no return is filed or a false and fraudulent return is filed with intent to evade the tax imposed by sections 143.011 to 143.996, a notice of deficiency may be mailed to the taxpayer at any time.

4. If a taxpayer fails to comply with the requirement of section 143.601 by not reporting a change or correction increasing his federal taxable income or by not filing an amended return, a notice of deficiency may be mailed to the taxpayer within one year after the director of revenue shall become aware of such determination. A notice under this subsection shall be limited to the effects on Missouri taxable income of:

(1) The issues on which the federal determination is based, and

(2) Any change in the amount of his federal income tax deduction under the provisions of subsection 1 of section 143.171.

5. If the taxpayer shall pursuant to section 143.601 report a change or correction or file an amended return increasing his federal taxable income or report a change or correction which is treated in the same manner as if it were a deficiency for federal income tax purposes, the assessment (if not deemed to have been made upon the filing of the report or amended return) may be made at any time within one year after such report or amended return was filed. A notice under this subsection shall be limited in the manner provided in subsection 4 of this section.

6. Where, before the expiration of the time prescribed in this section for the assessment of a deficiency, both the director of revenue and the taxpayer shall have consented in writing to its assessment after such time, the deficiency may be assessed at any time prior to the expiration of period agreed upon. The period so agreed may be extended by subsequent agreement in writing made before the expiration of the period previously agreed upon.

7. For purposes of this section a return filed before the last day prescribed by law or by regulation promulgated pursuant to law for the filing thereof shall be deemed to be filed on such last day. If a return for any period ending with or within a calendar year is filed before April fifteenth of the succeeding calendar year, such return shall be deemed to be filed on April fifteenth of such succeeding calendar year.