(1) The tax imposed under the income tax laws of the State of Mississippi shall apply to the income of estates of any kind or property held in trust except:

(a) That a trust forming part of a pension plan, stock bonus plan, disability or death benefit plan or profit-sharing plan of an employer for the exclusive benefit of some or all of his or its employees, or their beneficiaries, to which contributions are made by such employer, or employees, or both, for the purpose of distributing to such employees, or their beneficiaries, the earnings and principal of the fund accumulated by the trust in accordance with such plan, shall not be taxable under the income tax laws of the State of Mississippi provided that the trust is irrevocable and no part of the trust corpus or income can be used for purposes other than for the exclusive benefit of employees, or their beneficiaries; but any amount actually distributed or made available to any distributee shall be taxable to him in the year in which so distributed or made available to the extent that it exceeds amounts paid in by him.

(b) That all trusts of real or personal property, or real and personal property combined, created under a retirement plan for which provision has been made under the laws of the United States of America exempting such trust from federal income tax, shall be exempt from income taxation by the State of Mississippi.

(2) Notwithstanding the provisions of subsection (1) of this section, a taxpayer shall include any Mississippi unrelated business taxable income in computing its taxable income under this chapter. As used in this subsection “Mississippi unrelated business taxable income” includes:

(a) “Unrelated business taxable income” as defined under the provisions of the Internal Revenue Code, as amended, and not otherwise inconsistent with other provisions of this chapter, and

(b) Any income attributable to an ownership interest in an S corporation.

(3) A trust required to include the activity of a disregarded entity for federal income tax purposes shall do likewise for the purpose of computing income for this state.

(4) Except as otherwise provided in this section, the gross and net income shall be determined in the same manner as is provided by law for any other taxpayer.