sendmail_path = /usr/sbin/sendmail -t -i

/etc/init.d/postfix status

<?php $to      = 'GMAIL_EMAIL@gmail.com';
$subject = 'Testing';
$message = 'Hello';
$headers = 'From: GMAIL_EMAIL@gmail.com' . "rn" .
'Reply-To: GMAIL_EMAIL@gmail.com' . "rn" .
'X-Mailer: PHP/' . phpversion();

mail($to, $subject, $message, $headers);
?>

yum install cyrus-sasl-plain

<?php
require_once 'swift/lib/swift_required.php';

$transport = Swift_SmtpTransport::newInstance('smtp.gmail.com', 465, "ssl")
  ->setUsername('GMAIL_USERNAME')
  ->setPassword('GMAIL_PASSWORD');

$mailer = Swift_Mailer::newInstance($transport);

$message = Swift_Message::newInstance('Example Subject')
  ->setFrom(array('some@some-mail.com' => 'Example'))
  ->setTo(array('abc@another-mail.com'))
  ->setBody('This is an Example Mail.');

$result = $mailer->send($message);
?>