app/views/mailboxer/notification_mailer/new_notification_email.html.erb
<!DOCTYPE html>
<html>
<head>
<meta content="text/html; charset=UTF-8" http-equiv="Content-Type" />
</head>
<body>
<h1>You have a new notification: <%= @notification.subject.html_safe? ? @notification.subject : strip_tags(@notification.subject) %></h1>
<p>
You have received a new notification:
</p>
<blockquote>
<p>
<%= raw @notification.body %>
</p>
</blockquote>
<p>
Visit <%= link_to root_url,root_url %> and go to your notifications for more info.
</p>
</body>
</html>