Bob is unsure how to send emails so that no one can read it except the intended recipient.
Did you know that the body of emails can be encrypted? By exchanging public encryption keys, it is possible to encrypt the contents of an email so that only the people with the private key can decrypt the email contents. Now you can trade emails in secrecy, like spies! Note, however, that the metadata will not be encrypted. That is, who sent the email, who received it, when it was sent, and what the subject was, will NOT be encrypted. If you encrypted that part of the email, it would be impossible to deliver the email!
There are many tools that can do this, such as Enigmail. Try it out yourself and see how it works!