Generally speaking, sending an email over the internet is not secure. The message can be intercepted or even modified before it reaches the recipient. There are several technologies that can be used to provide point-to-point security. Two good options on the Mac are S/MIME, which is built into Mail.app, and Gnu Privacy Guard (GPG). GPG is based on the PGP standard, and can be used for free.
Both S/MIME and GPG can encrypt and digitally sign messages. Encryption will ensure that no one but the recipient can read the message. Digital signatures ensure that the email hasn’t been tampered with and wasn’t sent by an impostor.
A fundamental difference between S/MIME and GPG is how trust is established. With S/MIME, X.509 certificates are used. These certificates are typically signed by a commercial authority, but can also be self-signed. Businesses commonly use X.509 certificates. GPG uses keys that are signed by people who vouch for you. This is known as a web of trust. This mechanism is more decentralized and informal.
MacMaven Consulting can help you set up a email encryption mechanism for your home or office. Once installed, using secure email is incredibly simple. There are two buttons added to Mail, one for signing messages and another to encrypt them. MacMaven can also configure S/MIME on your iOS devices.