SMTP error from remote mail server after end of data: 550 High probability of spam with PHP mail()?

This is my mail

$to      = $email;
$subject = "Welcome To Website";
$header  = "from: Website <contact@website.com>";
$message  = "We have created your account on Website with username,".$username."";
$message .= ". To verify your account please click on the link - ";
$url = 'http://website.com/?code=' . $code . '';
$message .= $url;
$message .= ". After Logging in you will be able to access to additional features to make the most of your Website account. If you have any question you can send us a mail or a message on our official Facebook page.";
if (mail($to, $subject, $message, $header)) {
  return true;
}

Now, the mail returned true but it was not sent. When I looked further into the issue I found out that I get following error

DHE-RSA-AES256-SHA:256: SMTP error from remote mail server after end of data: 550 High probability of spam

How can I avoid this? I have tried to made the email as personalized as I can with available information. Is their some other library that I should use? Is there some way I can reword the mail? I am using Cloudflare service if that can create issues with DNS or something.

Thanks.