California Code, Elections Code - ELEC § 15106

Except as otherwise provided, the processing of vote by mail ballot return envelopes, the processing and counting of vote by mail ballots, and the disposition of challenges of vote by mail ballots shall be according to the laws now in force pertaining to the election for which they are cast. Because the voter is not present, the challenger shall have the burden of establishing extraordinary proof of the validity of the challenge at the time the challenge is made.