Now, I know that backup MXes (or is that MX backups?) have become somewhat questionable in the eyes of many a mail administrator, mostly because it seems quite unlikely today that a primary MX can be allowed to stay offline for more than say 24 straight hours.
After all, they do have someone doing IT there, don't they?
And I couldn't agree more, but you know, some businesses still like to think along the blessed lines of better safe then sorry and not my fault, so, as a consequence, I recently had to install and configure a backup MX server.