Idaho Code § 34-1009
The vote of any absent elector may be challenged in the same manner as other votes are challenged and the receiving judges shall have power and authority to determine the legality of such ballot. If the challenge be sustained, or if the receiving judges determine, that the affidavit accompanying the absent elector's ballot is insufficient, or that the elector is not a qualified registered elector the envelope containing the ballot of such elector shall not be opened and the judges shall endorse on the back of the envelope the reason therefor. If an absent elector's envelope contains more than one (1) marked ballot of any one (1) kind, none of such ballots shall be counted and the judges shall make notations on the back of the ballots the reason therefor. Judges of election shall certify in their returns the number of absent electors' ballots cast and counted and the number of such ballots rejected.
Idaho Code § 34-1009