app/views/user_mailer/threshold_exceeded_email.html.erb
<!DOCTYPE html>
<html>
<head>
<meta content='text/html; charset=UTF-8' http-equiv='Content-Type' />
</head>
<body>
<p>Your <%= @sensor %> measurements exceeded the value defined in your threshold alert for session <%= @title %>.</p>
</body>
</html>