You need to spread the word to users that they MUST clean up their mailboxes by COB on [date]. Anything older than [days] still in their mailboxes at that time will be permanently, irrecoverably purged. Require that they reply to the email acknowledging they understand their compliance is mandatory. Then, do a CYA backup of the mailstore followed by the purge.
It also sounds like your org needs to implement and enforce a formal records retention policy that covers both paper and electronic information. Even if you're a small fry in an unregulated industry, you never know when you could be slapped with a subpoena for your mail store, archives, backups, even BYODs. You don't even have to be the ones directly involved in litigation: A number of my clients have been subpoenaed by simple virtue of having exchanged electronic or snail mail on a regular basis on matters completely unrelated to the litigation. -- "Dance like the photo isn't being tagged; love like you've never been unfriended; and tweet like nobody is following."
I know. I was trying to stick to generics since a lot of folks these days seem to not be able to stretch concepts. I get a lot of "But you said such-and-such! You mean I also can do that with XYZ?" -- "Dance like the photo isn't being tagged; love like you've never been unfriended; and tweet like nobody is following."
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