the backup mx shouldn't have any special delivery privileges. when the primary mx comes back online, the backup should deliver to that rather than trying to deliver anything itself. this allows those spam checks which will still work to benefit from the normal delivery path.
you can have several hosts with the same MX priority, and only spin up actual service when necessary. given modern health check tools and raft-consensus filesystems, it's very possible to build a robuse mail network on the cheap.
you can have several hosts with the same MX priority, and only spin up actual service when necessary. given modern health check tools and raft-consensus filesystems, it's very possible to build a robuse mail network on the cheap.