邮件功能将HTML显示为文本

我有以下邮件功能：

<?php
{
$to = "alee@####.com";
$subject = "General Contact.";
$headers  = "From: ####<noreply@####.com>\r\n";
$headers .= "Reply-To: info@####.com\r\n";
$headers .= "Return-Path: info@####.com\r\n";
$headers .= 'MIME-Version: 1.0' . "\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
$name = filter_input(INPUT_POST, 'name');
$telephone = filter_input(INPUT_POST, 'phone');
$nature = filter_input(INPUT_POST, 'nature');   
$comments = filter_input(INPUT_POST, 'comments');
$message = 
"<span style='font-weight: bold;'>Name: </span>"."<br />".$name."<br />"."<br />".
"<span style='font-weight: bold;'>Telephone: </span>"."<br />".$telephone."<br />"."<br   />".
"<span style='font-weight: bold;'>Nature of enquiry: </span>"."<br />".$nature."<br />"."<br />".
"<span style='font-weight: bold;'>Comments: </span>"."<br />".$comments."<br />"."<br />";

mail($to, $subject,"We have received a message via the online form at www.####.com<br />  <br />" . $message, $headers);
echo "Your message was successfully sent. Please <a href='contact.php'>click here</a> to return to the site.";
?>
<?php  }
?>

我的问题是，当使用表单并且邮件打到我的收件箱时，HTML被显示为纯文本，而不能正确显示。因此，我可以看到所有内联HTML元素，
即将会显示为段落的一部分，而不显示新行。任何帮助都是极好的！

编辑：

Reply-To: info@###.com
Return-Path: info@###.com
MIME-Version: 1.0
Content-type: text/html; charset: utf8

We have received a message via the online form at www.###.com<br /><br /><span    style='font-weight: bold;'>Name: </span><br />Aaron Lee<br /><br /><span style='font-weight:   bold;'>Telephone: </span><br />#####<br /><br /><span style='font-weight:   bold;'>Nature of enquiry: </span><br />###<br /><br /><span style='font-weight:   bold;'>Comments: </span><br />Testttttttttttt<br /><br />