php - Code Igniter wont send emails to @hotmail addresses

I am using the following code to send my emails upone registration - it works for all email addresses except from @hotmail.com / .co.uk.

'$email_address = $_REQUEST['email'];
$message =  'Welcome Member, <br /><br />


<p><a href="'.base_url().'site/activate/'.$lastId.'/" target="_blank">'.base_url().'site/activate/'.$lastId.'/</a></p>

';

$AdminEmail  = '[email protected]';
$Subject  = 'Account Confirmation';

//@send_email($emailadderss,$Subject, $message, $AdminEmail, '');
$headers  = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
$headers .= 'From: [email protected]' . "\r\n";
$msg = @mail($email_address, $Subject, $message, $headers);
Copy code

I am sure why this is and I cannot find any solutions upon searching the internet.

Many thanks in advance.