var emails = []; // array
var number = 5;
var count = 0;
var dinamic_delay;
for (i = 0; i < emails.length; i++) {
    count++;
    if (count == number) {
        dinamic_delay = 3000;
        count = 0; // resset count
    } else {
        dinamic_delay = 500;
    }
    setTimeout(function (i) {
        sendemail(email[i]);
    }, dinamic_delay * i, i);
}