This catches any organisation that doesn't have centralised control over all emails.
First, when it's subpoenaed, you can't stop looking for them. "I can't find it" isn't an answer. "It's been destroyed" is the answer.
So you need to re-create the full body of emails on an email server, here's where you look, listed in order of importance and difficulty:
1) The email server.
2) Backups of the email server.
3) The email servers that talk to that server that you control.
4) The backups of those servers.
5) The individual PCs of the persons involved in the conversations.
6) The backups of those PCs.
7) Old, retired PCs in storage.
8) Any backups of those PCs.
If you, as an organisation are told by a court to find the emails, you hunt through _all_ the systems you control to find them. This is why organisations have centralised control over documents and emails with defined document destruction schedules. Otherwise, you get caught like Microsoft did in the Netscape trial where an email that was supposed to have been destroyed was found on someone's PC.
This does not mean that there was an intent to hide anything, only that it takes longer to build up the entire list.