I copied and pasted your two header lines into my code and then added the $headers variable into the mail function. I put one of my sentences as bold and when I received the e-mail, it was still in plain text.
This is how I bolded the line:
$message .= "\n<b>This message was sent by an account that does not accept e-mail. Do not try to reply - your messages will be denied.</b>";
And this is how I added the variable to the mail function:
mail ($to, $email_subject, $message, $from, $headers);
This is what I got in the e-mail for that line:
<b>This message was sent by an account that does not accept e-mail. Do not try to reply - your messages will be denied.</b>
What am I doing wrong?