Use encryption to protect your email messages. The only way to protect email messages from the interception is to encrypt them. There are few techniques to do so here is some info on types of encryption keys.
PGP and S\MIME encryption. Both PGP and S\MIME encryption are used to encrypt message body only, leaving message headers unprotected. PGP and S\MIME can be used if you require end-to-end encryption. Using those methods requires prior agreement between parties, and public key exchange should be done before emailing securely.
S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key encryption and signing of MIME data.
Pretty Good Privacy (PGP) is a data encryption and decryption computer program that provides cryptographic privacy and authentication for data communication.
A PKI (public key infrastructure) enables users of a basically unsecure public network such as the Internet to securely and privately exchange data and money through the use of a public and a private cryptographic key pair that is obtained and shared through a trusted authority. The public key infrastructure provides for a digital certificate that can identify an individual or an organization and directory services that can store and, when necessary, revoke the certificates. Although the components of a PKI are generally understood, a number of different vendor approaches and services are emerging. Meanwhile, an Internet standard for PKI is being worked on.
SSL encrypted connection to mail server. SSL can be successfully used to encrypt email traffic in the whole. SSL encrypted transport prevents from message headers and message body interception on the way to/from the mail server while sending/receiving email. SSL can be used to effectively protect from intercepting your email traffic by ISP or government agencies.
In addition to PGP or S/MIME, SSL connection encryption should be used to protect email message while in transit.