Define the datasource as a hardcoded list of emails

MainActivity.java

private void inbox() {
    emails = new ArrayList<>();
    Email email = null;
    for (int i = 0; i < 25; i++) {
        email = new Email(0, "Magda " + i, "Hello Android " + i, "This is an intro about Android");
        emails.add(email);
    }
}