Most organizations are unclear about the retention policies they should adopt. That slows the deployment of archiving technology.
It usually makes sense to go ahead with archiving in advance of having decided one’s retention policy; that is, start by archiving everything, for an indeterminate period. You still get many of the advantages of archiving, such as better user mailbox performance, better e-discovery, and reduced backup times. As you go along, you can reduce the scope of archiving (e.g., don’t include spam) and define retention policies.