app/views/admin/mailer_previews/index.html.erb
<% content_for :page_title, 'Email previews' %>
<h1>Email previews</h1>
<% ActionMailer::Preview.all.each do |mailer| %>
<strong><%= mailer.name.underscore.humanize %></strong>
<ul>
<% mailer.emails.each do |email| %>
<li><%= link_to email, admin_mailer_preview_path(mailer.preview_name + '/' + email) %></li>
<% end %>
</ul>
<% end %>