Shall not be exempt from taxation under section 501(a) for any taxable year subsequent to the taxable year in which there is mailed to it a notice in writing by the Commissioner that it has engaged in such prohibited transactions. Such notification by the Commissioner shall be by registered or certified mail to the last known name and address of the organization. However, notwithstanding the requirement of notification by the Commissioner, the exemption shall be denied with respect to any taxable year if such organization during or prior to such taxable year commenced the prohibited transaction with the purpose of diverting income or corpus from its exempt purposes and such transaction involved a substantial party of the income or corpus of such organization. For the purpose of this section, the term taxable year means the established annual accounting period of the organization; or, if the organization has no such established annual accounting period, the taxable year of the organizations means a calendar year. See 26 CFR § 1.503(j)-1 (rev. as of Apr. 1, 1974) for provisions relating to the definition of prohibited transactions in the case of trusts benefitting certain owner-employees after December 31, 1962, but prior to January 1, 1975. See also section 2003 (c)(1)(B) of the Employee Retirement Income Security Act of 1974 (88 Stat. 978) in the case of an organization described in section 401(a) with respect to which a disqualified person elects to pay a tax in the amount and manner provided with respect to the tax imposed by section 4975 of the Code so that the organization may avoid denial of exemption under section 503. For further guidance regarding the definition of last known address, see § 301.6212-2 of this chapter.
26 C.F.R. §1.503(a)-1