There is unfortunately no way to be sure: There are so many spam filters out there. BallotBox follows best practices to try to minimize the incorrect identification of our emails as spam, but there is no way to ensure it never happens.
The single best thing you can do is to encourage your voters to put noreply@ballotboxonline.com into their email contacts, or otherwise to configure their spam filters to allow all email from that specific address, by directing them to this support article. Many spam filters automatically “bless” all emails that's are coming from an address in the recipient’s contact list.