When creating a new message, you will find the options for encrypting and signing under „Options“. Signing should be set by default, so you don't need to do anything else.
To encrypt, you must select the „Encrypt“ option. You can only encrypt if you have received the recipient's public key. Don't worry: if you press „Send“ and the recipient's key is not available, an error message will appear and the message will not be sent in plain text.
How do you get the recipient's key?
Ask the recipient to send you a message signed by them. This contains their public key and you can reply to them in encrypted form. If you enter the recipient as a contact in Outlook (right-click on the sender address of the signed e-mail you have received), the public key will be entered in your address book and you can always send encrypted messages to this recipient from now on.
You only have to do this process once, from then on the recipient's public key is saved on your computer.
ATTENTION: The subject of the message is not encrypted. Do not pass on any confidential data in the subject line. However, attachments are encrypted.
Received messages are decrypted automatically. You will only see a small lock next to the message in the list. Even if you move the message to another directory in Outlook, it remains encrypted. It is only displayed in plain text in the window of your mail client.
Deutsche Version: Email verschlüsseln