Cal. Revenue and Taxation Code § 3799

The tax collector shall mail a copy of the notice not less than 45 nor more than 60 days prior to the effective date of the agreement, by registered mail to the last assessee of each portion of the property and to parties of interest, as defined in Section 4675, at their last known address. To ascertain the address of the last assessee of the property an examination shall be made of the assessment of this property on the rolls beginning with the year of delinquency to and including that of the last equalized roll. The tax collector shall make reasonable efforts to ascertain the identity and address of parties of interest. It is not necessary to mail a copy of the notice to any party who files with the tax collector a written acknowledgment of receipt of a copy of the notice or a waiver of the notice. The validity of any sale under this chapter shall not be affected if the tax collector’s reasonable effort fails to disclose the name and last known mailing address of parties of interest or if a party of interest does not receive mailed notice.