How to set up "backupMailMX" flag?

==== Required information ====
- iRedMail version:
- Store mail accounts in which backend (LDAP/MySQL/PGSQL):
- Linux/BSD distribution name and version:
- Related log if you're reporting an issue:
====

Where is this flag located? I did not see it in LDAP.

I want to use IRedMail LDAP, but I do not want mails to be delivered locally as I use other mail provider for the actual e-mail.

Thanks