Email is the electronic way to deliver information that used to be delivered by mail or fax.
It is fast, cheap and convenient. This problem exists for normal email messages and attachments to email messages. The only sure way to 'seal the envelope' is by scrambling the contents so that no one except the person the message is intended for can unscramble it. The password must be known by both the sender and the recipient. Most encryption programs are quite technical and require a fair degree of computer literacy to use effectively.