Emails are not sent to outside domains I deployed a Drupal 8 website, on a hosting service with cPanel, with domain mydomain.com. 

If I set this email address on the user profile: john.smith@example.com, that is an email address with a domain different from mydomain.com, then I go to:

mydomain.com/user/password


and I click to "Send" trying to receive the "reset password" email, it give me the error below and I don't receive the email:


  Unable to send e-mail. Contact the site administrator if the problem
  persists.


Same problem on contact forms.

If I set an email address within my domain, for example john.smith@mydomain.com, all works and the email is correctly sent.

Any idea why this is happening and how to solve this problem?



NOTE

I tried to create this php script inside the server in order to check if there is some issue on the server configuration:

&lt;?php

$to = "john.smith@example.com";
$subject = "Test mail";
$message = "This is a test message.";

$headers = [];

$headers[] = "From: noreply@example.com";
$headers[] = "MIME-Version: 1.0";
$headers[] = "Content-type: text/plain; charset=UTF-8; format=flowed; delsp=yes";
$headers[] = "Content-Transfer-Encoding: 8Bit";
$headers[] = "X-Mailer: Drupal";
$headers[] = "Reply-To: noreply@example.com";

$res = mail($to, $subject, $message, implode("\n", $headers));

echo "Mail Sent. &lt;br/&gt;\n";
echo "Res: " . $res . " &lt;br/&gt;\n";


When I run this script it correctly works and sends the email, either if the $to property is from my domain or if it is from an external domain.

So I excluded it is a server configuration issue.
I solved the problem.

Taking inspiration from this thread, I figured out that, for some obscure reason, I need to set the email setting under "Administration > Configuration > System > Basic site settings > E-mail address" to an existing email address on my hosting service.

So, I created a new email with cPanel, noreply@mydomain.com, then I setted this email address in the above configuration.

Now seems that all is working perfectly.