Your question doesn't directly indicate which problem you are trying to solve. It would be helpful if you would mention the hosting provider as their service of

Your question doesn't directly indicate which problem you are trying to solve. It would be helpful if you would mention the hosting provider as their service offerings do differ. If you are trying to solve storage reliability for single mail server then creating a file system level failover would be way to go as mentioned previously (LVM, soft RAID over multiple volumes, etc). If you are trying to solve mail service reliability/failover then adding second mail server would be helpful. In that case you can use dsync (does master/master sync) to keep emails on both servers in sync.