SMTP, POP3 and IMAP are plain text protocols. This means a sniffer on the network can read all the messages. This is, of course, a very bad thing to happen when sensitive messages are transmitted.
To stop sniffers, SMTP, POP3 and IMAP can tunnel through TLS (transport layer security), formerly known as SSL (secure socket layer). TLS traffic is encrypted, which makes it very difficult for sniffers to read the content of messages.