I wouldn't call it "the most secure", there is no id check (because there is no mandatory ID scheme) so the process to verify your identity is super loose. Also

I wouldn't call it "the most secure", there is no id check (because there is no mandatory ID scheme) so the process to verify your identity is super loose. Also very little constraints on voting by mail, with the only form of authentication being a signature which is weak to say the least. France has much stronger controls on ID checks at the voting booth and vote by mail (in fact for most elections there is no vote by mail, you can only delegate your vote to a person who cannot act as a delegate to more than two people to prevent ballot harvesting).