Cal. Elec. Code § 15105

Prior to processing and opening the identification envelopes of vote by mail voters, the elections official shall make available a list of vote by mail voters for public inspection, from which challenges may be presented. Challenges may be made for the same reasons as those made against a voter voting at a polling place. In addition, a challenge may be entered on the grounds that the ballot was not received within the time provided by this code or that a person is imprisoned for a conviction of a felony. All challenges shall be made prior to the opening of the identification envelope of the challenged vote by mail voter.