The Path to E-Mail Production
This article has a brief but exceptionally helpful list of sources for email on computer systems. The most basic gut reaction is to simply log on as an individual and print out their emails from Outlook, Lotus Notes, or whatever email program the person uses. With a little more thought, you would actually check the files on the email server, search them, and save them to an electronic file. Even that will often overlook significant treasure troves of email. Whether you are requesting or responding to discovery of email, it's vital you consider and look to all of the reasonably accessible sources.
There's an old saying: If it's worth doing, it's worth doing right. Print out and keep a copy of the linked article so you can manage discovery - and production - of email appropriately.