This method has been used for years and it works. The problem: it's not very easy for a non-technical person to implement it.
Conceptually, the sender signs an email with the recipient's public key. The only way to decrypt this encrypted email is
now with the recipient's private key. Since no one but the recipient should have access to his/her private key, the message
stays encrypted throughout its journey.
Problems - To complicated to implement
- Most non-technical users do not know what public/private keys are
- How will the sender get recipient's public key
- Must use email clients that are capable of using OpenPGP. For example, may not work with certain mobile devices
Because of the complexities in creating and obtaining public/private keys, this method is not used widely in the industry.