during the 9 ½ -month period beginning on the first date any of the persons referred to in subsection (e) knew that such failure existed, or exercising reasonable diligence would have known. (3) Overall limitation for unintentional failures In the case of failures which are due to reasonable cause and not to willful neglect— (A) General rule The tax imposed by subsection (a) for failures during the taxable year of the employer shall not exceed $500,000. (B) Taxable years in the case of certain controlled groups For purposes of this subparagraph, if not all persons who are treated as a single employer for purposes of this section have the same taxable year, the taxable years taken into account shall be determined under principles similar to the principles of section 1561. (4) Waiver by Secretary In the case of a failure which is due to reasonable cause and not to willful neglect, the Secretary may waive part or all of the tax imposed by subsection (a) to the extent that the payment of such tax would be excessive relative to the failure involved. (d) Tax not to apply in certain cases This section shall not apply in the case of— (1) any employer that employed no more than 10 employees each of whom received at least $5,000 of compensation from the employer during the prior calendar year, (2) any employer with respect to a governmental plan (within the meaning of section 414(d)), (3) any employer with respect to a church plan (within the meaning of section 414(e)), or (4) any employer that has been in existence for fewer than 2 years, taking into account all predecessor employers. (e) Liability for tax The employer shall be liable for the tax imposed by subsection (a) on a failure. All employers, determined without regard to subsection (f)(2), shall be jointly and severally liable for the liability of any other employer with which they are aggregated under subsection (f)(2). (f) Definitions For purposes of this section— (1) Automatic contribution plan or arrangement The term automatic contribution plan or arrangement