California Code, Insurance Code - INS § 666

Where the reason for cancellation does not accompany or is not included in the notice of cancellation, the insurer shall upon written request of the named insured, mailed or delivered to the insurer not less than 15 days prior to the effective date of cancellation, specify in writing the reason for the cancellation.The reason shall be mailed or delivered to the named insured within five days after receipt of the request.Notwithstanding Section 1633.3 of the Civil Code, the writing specifying the reason for cancellation may not be delivered electronically to the insured unless the insured electronically delivered the request to the insurer.