Today, we have a product called IBM Compliance for mail which does a simple fetch all mail to an archive database. It is sort of affordable, but does not deliver two very important features:
1) accessibility of the archive
The archive is a web app that allows you to define auditor users to see mails that fulfill a certain rule set. This way it is possible (but nearly impossible to handle) to create a user access for every user in the organization to their own archived mails by defining mail search rules like (TO email@example.com OR from firstname.lastname@example.org OR CC to email@example.com OR BCC to firstname.lastname@example.org ) and the create a user bgewehr that is allowed to see the search results of that rule.
Now repeat that for the amount of users you have and care for all changes happening with renaming, retiring, leaving and entering of people and your're done with an archive that is A) compliant and B) helps users find old stuff. Possibe to do but impossible at scale.
2) completeness of the archive
On of the informations that people NEED to have in the operative mailbox as well as in the archive is the context that a mail is assigned to. People have no tagging in mail so the only way to add context to mail is to put that mail in a context giving folder. In the archive of IBM compliance for mail this context gets lost, the mail is put into the archive way before a user adds a context. It should get the information of context from the users actions and reflect these in the archive as well. (Compliance for mail listens to Webhook from domino telling which document was put into which folder and then do the same with the email in the archive without changing its content for compliant, unchanged archiving)