When you sign up for the backup mail service, we automatically add our backup mail server in an MX record to your zone. This causes all incoming mail to attempt your primary server first, and if that fails, to try our backup server. When our backup server gets mail for you, it looks up your primary server, and periodically attempts to redeliver your mail to the primary. It will attempt redelivery for 10 days before returning the mail to the sender as undeliverable.