Don't confuse your backup system for an email archive. Relying on backup as a way to quickly search email records can be a costly mistake.
Backup is all about quickly recovering a file or a damaged server.
Archive is about search, finding the "needle in the hay stack" quickly.
Two very different needs and therefore different solutions are needed. Implement a cloud compute based hosted email archive solution to get the best ROI for your IT budget.
From the CMS Watch article:
For the record: a back-up is designed to manage short-term risk and provide a facility for disaster recovery. An archive, on the other hand, is designed to help manage long-term risk, ensuring that historical data can be accessed and remains authentic either for the business user or even an auditor or lawyer.