Why my html code also display when i send email?

hi dear this is my code to send email
my message code is this

$to = $email;
 $subject = "Activation";
 $message = "Your activation key is this " .$key.'<br>'.' click here to activate your acount. <a href="activation.php?key='.$key.'">here</a>';
 $from = "riaz_qadeer90@yahoo.com";
 $headers = "From:" . $from;
 if(mail($to,$subject,$message,$headers))
 {
     echo "Check your email to activate your acount.";
 }
 

The problem is this when i send email the whole message shown in my inbox with code. why it not show "Click" as an anchore.....

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