Could you verify your definition of what you consider 'archiving'?
I want to make sure your not confusing archiving & discovery for history/compliance purposes/cross mailbox search with hierarchical storage management (HSM) which allows you to configure storage volumes for older messages. (Ie: Automatically move older messages to cheaper storage solutions, all totally transparent to the user.)
Both are Network Edition features-Zimbra support can help you implement it.
/products/zimbra_archiving.html /pdf/Zimbra Archiving and Discovery Whitepaper.pdf /pdf/Zimbra Archiving and Discovery Release Notes.pdf
Poor man's 'compliance' would be to always send copies of emails to a particular address using always_bcc &/or forwarding.