Something link that could be useful as disaster recovery action. I case of problems with the Mail infrastructure redirect the DNS to something like that and incoming e-mails won't get lost in the meantime. The standby cost for something like that is much lower then a independent secondary e-mail system.
Why redirect anything? You can have backup MX records. This is already a solved problem. And backup system can be quite lightweight, if it only needs to store emails for the time primary is down and then push them to primary once it's up again.
