It's possible a server hard drive failed, but the overwhelming majority of places back up to tape. Tape very very rarely fails.
Also, when you send an email, there's at least two copies. On the Sent folder of the sender, and the Inbox of the receiver. Multiple receivers, multiple copies. It's very rare for a server RAID to fail. It's more rare at a very large organization for the sender and receiver to be on the same server RAID (not the case at smaller organizations with only one email server). It's very rare that organizations do not back up to long term media.
Only alternative I ever see is if an entity has a very aggressively short retention policy mandating destruction in a short period of time.