Introduction
/etc/backupmxhosts contains a list of remote servers that will behave as secondary mx hosts for your mail servers. This allows them to bypass sender rate limits.
Procedure
This file can be modified via WHM at the following location.
WHM >> Service Configuration >> Exim Configuration Manager >> Access Lists >> Backup MX Hosts
Please see the following documentation for more information.
https://docs.cpanel.net/whm/service-configuration/exim-configuration-manager-basic-editor/