add validator for the 'reply_by_email_enabled' site setting

lib/validators/reply_by_email_enabled_validator.rb

@@ -0,0 +1,23 @@
+class ReplyByEmailEnabledValidator
+
+  def initialize(opts={})
+    @opts = opts
+  end
+
+  def valid_value?(val)
+    # only validate when enabling reply by email
+    return true if val == "f"
+    # ensure reply_by_email_address is configured && polling is working
+    SiteSetting.reply_by_email_address.present? &&
+    SiteSetting.pop3_polling_enabled?
+  end
+
+  def error_message
+    if SiteSetting.reply_by_email_address.blank?
+      I18n.t("site_settings.errors.reply_by_email_address_is_empty")
+    else
+      I18n.t("site_settings.errors.pop3_polling_disabled")
+    end
+  end
+
+end