Email encryption refers to encryption and authentication of email messages, which is typically done to protect content from being read by unintended recipients.
Email technology is readily available but has yet to achieve widespread usage, most likely due to social factors, such as people seeing the use of encryption as paranoid, and as marking a message urgent whether it is or not. A recent survey (from January 2012) found that only 35 percent of companies have implemented email encryption.
Email encryption relies on public-key cryptography, in which users can each publish a public key that others can use to encrypt messages to them; while keeping secret a private key they use to decrypt such messages or to digitally encrypt and sign messages they send.
Popular protocols for email encryption are: PGP, S/MIME, TLS, Identity-based encryption, and Mail sessions encryption. Of these, Mail sessions encryption is the least reliable, and is easily tampered with by involved email relays. In other words, the encryption takes place between individual SMTP relays, and not between the sender and the recipient.