You just received an email from Bob, your eccouncil video

Full Certification Question

You just received an email from Bob, your investment banker, stating that he completed the wire transfer of $10,000 to your bank account in Vietnam. The problem is, you do not have a bank account in Vietnam, so you immediately call Bob to ask what happened. Bob explains that he received an email from you requesting the transfer. You insist you never sent that email to Bob initiating this wire transfer. What aspect of PKI could be used to BEST ensure that a sender actually sent a particular email message and avoid this type of situation?