Without encryption, email is vulnerable to packet-sniffing software designed in detect keywords in messages for the purpose of privacy invasion. Encryption helps protect against keyword searches because keywords are much harder to find in encrypted text.
Email is highly persistent.
The persistence problem is a huge security problem. Email sits on servers until delivered. The message may take hours to be delivered, bouncing across many servers before reaching the recipient. At any given point in this process the message may be archived. The email path can be traced.