PGP and S/MIME are popular end-to-end encryption standards used to encrypt emails in a way that no one, not even the company, government, or cyber criminals, can spy on your communication. The vulnerabilities exist in the way encrypted email clients handle HTML emails and external resources, like loading of images, styles from external URLs. To perform an eFail attack, an attacker must have access to your encrypted emails, which is then modified in the following way to trick your email client into revealing the secret message to the remote attacker.
Source: https://thehackernews.com/2018/05/efail-pgp-email-encryption.html