If you would like to encrypt e-mail for a recipient, you will need the public key of his/her Digital ID (also called digital certificate). Digital IDs are a special type of security document that helps to validate your identity. They can be used to sign or encrypt important documents electronically. The Digital ID has two parts: a private part that only you use and a public key. The private key must be kept securely, preferably on a secure device like a smart card.

To send an encrypted email to someone, you will need to include that person's public key. On reception of the secure message, your correspondant will use his/her private key to decrypt (or "unscramble") the message. No one without the private key will be able to read the e-mail.

See also, How do I know if the emails I send from my personal computer at home are encrypted?