Five years of email gone?
You went to THE backup??
You made a single backup in 5 years -- and re-writing over and over and over on the same backup file?
You know, it's a good practice to make frequent backups and have each one uniquely and sequentially identified so that if you lose one, you can go to another backup. You might (possibly) lose some information -- but losing 5 years of email because a single file had a problem? Ever consider RAID for reliability?
It's also good practice to test the backup system frequently.
Working around safety systems, I learned the principle that you don't want to learn that your safety system has failed by having it fail to respond to a dangerous situation.
The solution to this problem is to test the system frequently to verify that it is operating. If you test the system 10 times more frequently than you have real issues, then you increase the probability that you will discover a safety system failure without allowing a dangerous situation.
The same principle applies to computer backups. If you never test them, you will always detect a backup system problem by being unable to provide a backup.