PHP mail() does not work on digitalocean ubuntu 16.04 server

Question

my tools: server= digitalocean domain= godaddy installed apache/2.4.18 php 7.0.30 ubuntu 16.04

i have tried install sendmail and configure it as below:

sudo apt-get install sendmail

and configure the /etc/hosts

sudo nano /etc/hosts

i am sure it contains 127.0.0.1 localhost localhost.localdomain myhostname and i made

sendmailconfig

yes for all questions in sendmail config. also restarted apache2 with

sudo service apache2 restart

it is not sending mail. can u help me?

i tried send mail with send.php it contains:

<?php
$to = 'sendto@outlook.com';
$title = 'title of mail';
$content = 'hello from world';
$titles = 'From: sendfrom@outlook.com' . "\r\n" .
    'Reply-To: sendfrom@outlook.com' . "\r\n" .
    'X-Mailer: PHP/' .phpversion();
mail($to, $title, $content, $titles);
?>

Answer 1

Digitalocean has shared ip for users therefore, outlook, gmail etc. put it in the blacklist. In summary, mail() function works but does not deliver the mails.