public static void main(String[] args) {

    List<String> emails = Lists.newArrayList();
    emails.add("first-person@gmail.com");
    emails.add("second-person@gmail.com");
    emails.add("third-person@gmail.com");
    emails.add("fourth-person@gmail.com");
    emails.add("fifth-person@gmail.com");
    emails.add("sixth-person@gmail.com");
    emails.add("seventh-person@gmail.com");
    emails.add("eighth-person@gmail.com");
    :
    :

    for(String email: emails) {
        sendMail(email);
    }
}
private static void sendMail(final String email) {
    // function will call many other services for getting the information required for sending the email
    try {
        Thread.sleep(10000);
        System.out.println("ownerEmailDetail====>"+email);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}

public static void main(String[] args) {

    List<String> emails = Lists.newArrayList();
    emails.add("first-person@gmail.com");
    emails.add("second-person@gmail.com");
    emails.add("third-person@gmail.com");
    emails.add("fourth-person@gmail.com");
    emails.add("fifth-person@gmail.com");
    emails.add("sixth-person@gmail.com");
    emails.add("seventh-person@gmail.com");
    emails.add("eighth-person@gmail.com");
    :
    :

    emails.stream().parallel().forEach(email -> {
        System.out.println("given==>"+email);
        sendMail(email);
    });
}
private static void sendMail(final String email) {
    // function will call many other services for getting the information required for sending the email
    try {
        Thread.sleep(10000);
        System.out.println("ownerEmailDetail====>"+email);
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}