If you plan on sending or receiving credit card information via e-mail, you will need to encrypt the information that is sent.
PGP, which stands for "pretty good privacy," is the most common form of e-mail encryption. PGP encrypts e-mails when they are sent and decrypts when the information has reached its intended recipient. Make sure your e-mail clients supports PGP and keep your security key in a location where it cannot be accessed by anyone else.