Add a way to show a custom message at the top of all notification emails. Customize it at /admin/site_contents/notification_email_top

app/mailers/user_notifications.rb

@@ -192,10 +192,12 @@ class UserNotifications < ActionMailer::Base
       end
     end
 
+    top = SiteContent.content_for(:notification_email_top)
+
     html = UserNotificationRenderer.new(Rails.configuration.paths["app/views"]).render(
       template: 'email/notification',
       format: :html,
-      locals: { context_posts: context_posts, post: post }
+      locals: { context_posts: context_posts, post: post, top: top ? PrettyText.cook(top).html_safe : nil }
     )
 
     template = "user_notifications.user_#{notification_type}"

app/models/site_content.rb

@@ -24,6 +24,7 @@ class SiteContent < ActiveRecord::Base
   add_content_type :top, allow_blank: true, format: :html
   add_content_type :bottom, allow_blank: true, format: :html
   add_content_type :head, allow_blank: true, format: :html
+  add_content_type :notification_email_top, allow_blank: true, format: :markdown
 
   def site_content_type
     @site_content_type ||= SiteContent.content_types.find {|t| t.content_type == content_type.to_sym}

app/views/email/notification.html.erb

@@ -1,4 +1,10 @@
 <div id='main'>
+
+<% if top.present? %>
+  <div><%= top %></div>
+  <hr>
+<% end %>
+
 <%= render :partial => 'email/post', :locals => {:post => post} %>
 
 <% if context_posts.present? %>

config/locales/server.en.yml

@@ -578,6 +578,9 @@ en:
     tos_signup_form_message:
       title: "Signup Form: Terms of Service Message"
       description: "The message that will appear beside a checkbox on the signup form if the tos_accept_required site setting is enabled."
+    notification_email_top:
+      title: "Notification Email Top"
+      description: "A message that will be displayed at the top of all notification emails."
 
   site_settings:
     default_locale: "The default language of this Discourse instance (ISO 639-1 Code)"