With the original design of email protocol, the communication between email servers was in plain text, which posed a huge security risk. Over the years, various mechanisms have been proposed to encrypt the communication between email servers. Encryption may occur at the transport level (aka "hop by hop") or end-to-end. Transport layer encryption is often easier to set up and use; end-to-end encryption provides stronger defenses, but can be more difficult to set up and use.
