check json availablility
function isUniqueEmail(email) {
    $.getJSON('info/mail.json', function(json) {
        var response = json.mail
            if(response == email){
                alert("Your email is already on our list");
            }

            else {
                emailWrite(email);
            };
    });
};

{"mail":
[
    "[email protected]",
    "[email protected]",
    "[email protected]"
]}

function isUniqueEmail(email) {
    $.getJSON('info/mail.json', function(json) {
        var response = json.mail
            $(json).each(function(){
                if(this.mail == email){
                    alert("already gotcha")
                }
                else {emailWrite(email)
                }
            });
    });
};

$(json).each(function(){
    if(this.mail == email){
        // already on list
    }else{
        // writeEmail
    }
});