[jmah]

Parent is saying that the content of email is almost always transmitted in the clear. While the connection from client to server is (hopefully) secure, the server can see the plaintext of all messages passing through it — which it helpfully uses to provide some server-side facilities like search and filtering — but if someone hacks the server they get all the mail.

Contrast with more recent messaging protocols (e.g. iMessage, WhatsApp) where the server doesn't have access to the message, they're decrypted on the client (while potentially also authenticating the sender).