That is great for ongoing criminal investigations, but seldom of any use to civil litigators, as few ISPs keep backups of client e-mail for more than 30 days. But it might be worth asking. Groupware is designed to share and distribute documents within an office or among a group of colleagues around the world. Documents are drafted, commented on, and redrafted within the group. The software keeps track of every comment and version.
Some products even keep digital transcripts of conference calls.
Find out if the other side uses groupware or version-tracking software. Better yet, find out if your client does. In spite of recent publicity about the dangers of e-mail, people still become stupid once they get online.