The only addressing RONR has to email voting is to say that it is not allowed unless your bylaws specifically allow it.
RONR authorship promises to address conducting business by electronic means in their 11th edition. For now, the NAP (National Assoc. of Parliamentarians) has recommendations for conducting electronic meetings. Word of caution: Email voting is not the same as e-meetings.

Roberts Rules is a very good guide, but it won't address email voting unless you're reading a very, very new revision of Major Robert's 1876 rules. RRO assumes voting is done in person, unless the rules of absentee voting are expliciting spelled out in the bylaws. You have to consider all the possible complications with absentee voting. Can you explain the situation that has caused your PTO to evaluate email voting?

Do you have the emails of all your families registered somewhere with the PTO. Otherwise, how would you know that one person with multiple email accounts at home (various family members) is not voing more than once and signing off as someone else?

We have a proxy form that parents can use if they cannot attend the vote (just like in all US elections). They only get one form which is registered and must be accounted for as returned for it to count. Otherwise, parents need to show up to cast their vote.

Our PTO is trying to conduct email voting. Does anyone have any suggestions on how to do this? It is not addressed in our by-laws. I am going to go buy Robert's Rules tomorrow.