I'm trying to send automated emails with mail(). It sends some emails but not all, around 50%. To test I'm using the same email address for all emails, and still only some get delivered.

I'm using localhost XAMPP.

Here's the code:

if($_POST['sendEmail'] == "SEND Email"){

ob_start();

$buffer = str_repeat(" ", 4096); $buffer .= "\r\n some HTML \r\n";

set_time_limit(0);
$noEmails = $last - $first + 1;
echo "Emails sent (of $noEmails):";
for($index = $first; $index <= $last; $index++){
    $to = $email["$index"];   
    $subject = "Hey {$firstName["$index"]}!";
    $message = "$emailMessage";
    $headers = 'From: [email protected]' . "\r\n" .
    'Reply-To: [email protected]' . "\r\n" .
    'X-Mailer: PHP/' . phpversion();
    sleep(1);
    mail($to,$subject,$message,$headers);
    echo $buffer.$index;
    ob_flush();
    flush();
}   
ob_end_flush();

}

Please give your suggestions.