In PKI data encrypted using a public key can only be decrypted by the public key's private pair. James needs to send the email so only Bob can decrypt it, so he will use Bob's public key. When Bob gets the email he will use his private key to decrypt it, therefor ensuring confidentiality.
Email encryption is encryption of email messages to protect the content from being read by entities other than the intended recipients. Email encryption may also include authentication.
Email is prone to the disclosure of information. Most emails are currently transmitted in the clear (not encrypted) form. All emails sent using Gmail or Outlook are encrypted by default. It is possible to send an unencrypted email using Gmail but a warning icon will appear in the address …