Cal. ELEC § 15105

Challenges to a vote by mail voter 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 currently serving a state or federal prison term for the conviction of a felony. All challenges shall be made prior to the opening of the identification envelope of the challenged vote by mail voter.