My office has issued a guideline on this subject, to help people better understand their responsibilities in relation to emails. It is entitled "9 Tips for ATIP-Friendly Email Management".
We find that this is a problem even in our own office, in fact. We often allow employees a half-day to clean up their email inboxes because we have run out of memory. That should not be something that should be required; employees should do it systematically.
For example, if five of you are exchanging emails, we recommend that one person, the author of the original email, retain the chain of emails. Otherwise, if an access to information request is made and the five people have kept the chain, the five of them will have to respond to the request and you will have the same email exchange five times.
It would be preferable to establish clearer rules concerning who should keep emails and what documents are considered to be transitory. This does not mean deleting all the content in email inboxes; it means filing the necessary documents on a drive where they can be retrieved, instead of putting it on your own drive, where no one but yourself will have access in your absence.