Enable reply by email for all users, display a message in the footer indicating so.

app/mailers/user_notifications.rb

@@ -39,7 +39,7 @@ class UserNotifications < ActionMailer::Base
                 private_message_from: post.user.name,
                 from_alias: I18n.t(:via, username: post.user.name, site_name: SiteSetting.title),
                 add_unsubscribe_link: true,
-                allow_reply_by_email: user.admin?
+                allow_reply_by_email: true
   end
 
   def digest(user, opts={})
@@ -102,9 +102,6 @@ class UserNotifications < ActionMailer::Base
     username = @notification.data_hash[:display_username]
     notification_type = Notification.types[opts[:notification].notification_type].to_s
 
-    # For now only admins can reply by email
-    opts.delete(:allow_reply_by_email) unless user.admin?
-
     email_opts = {
       topic_title: @notification.data_hash[:topic_title],
       message: @post.raw,

config/locales/server.en.yml

@@ -900,6 +900,9 @@ en:
       title: "Unsubscribe"
       description: "Not interested in getting these emails? No problem! Click below to unsubscribe instantly:"
 
+    reply_by_email: "You can reply directly to this email to respond, or visit %{base_url}%{url} to reply in your browser."
+    visit_link_to_respond: "Please visit this link to respond: %{base_url}%{url}"
+
     user_invited_to_private_message:
       subject_template: "[%{site_name}] %{username} invited you to a private message '%{topic_title}'"
       text_body_template: |
@@ -916,7 +919,7 @@ en:
         %{message}
 
         ---
-        Please visit this link to respond: %{base_url}%{url}
+        %{respond_instructions}
 
     user_quoted:
       subject_template: "[%{site_name}] %{username} quoted you in '%{topic_title}'"
@@ -927,7 +930,7 @@ en:
         %{message}
 
         ---
-        Please visit this link to respond: %{base_url}%{url}
+        %{respond_instructions}
 
     user_mentioned:
       subject_template: "[%{site_name}] %{username} mentioned you in '%{topic_title}'"
@@ -938,7 +941,7 @@ en:
         %{message}
 
         ---
-        Please visit this link to respond: %{base_url}%{url}
+        %{respond_instructions}
 
     user_posted:
       subject_template: "[%{site_name}] %{username} posted in '%{topic_title}'"
@@ -949,7 +952,7 @@ en:
         %{message}
 
         ---
-        Please visit this link to respond: %{base_url}%{url}
+        %{respond_instructions}
 
     digest:
       why: "Here's a brief summary of the discussion on %{site_link} since we last saw you on %{last_seen_at}."

lib/email/message_builder.rb

@@ -39,6 +39,16 @@ module Email
       @template_args ||= { site_name: SiteSetting.title,
                            base_url: Discourse.base_url,
                            user_preferences_url: "#{Discourse.base_url}/user_preferences" }.merge!(@opts)
+
+      if @template_args[:url].present?
+        if allow_reply_by_email? and
+          @template_args[:respond_instructions] = I18n.t('user_notifications.reply_by_email', @template_args)
+        else
+          @template_args[:respond_instructions] = I18n.t('user_notifications.visit_link_to_respond', @template_args)
+        end
+      end
+
+      @template_args
     end
 
     def build_args

lib/jobs/user_email.rb

@@ -53,7 +53,6 @@ module Jobs
       raise Discourse::InvalidParameters.new(:type) unless UserNotifications.respond_to?(args[:type])
 
       message = UserNotifications.send(args[:type], user, email_args)
-
       # Update the to address if we have a custom one
       if args[:to_address].present?
         message.to = [args[:to_address]]