Backup MX & Relay setting

==== Required information ====
- iRedMail version: 0.8.4
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Linux/BSD distribution name and version: 5.8
- Related log if you're reporting an issue:
====
Hello Zhang
Can you show me step by step how to setting Backup MX & Relay email
I could not found any documents of iRedAdmin relating this 2 features

Thanks for answers !