Explanation/Reference:
Explanation:
Sending and viewing encrypted email messages requires both sender and recipient to share their digital ID, or public key certificate. This means that you and the recipient each must send the other a digitally signed message, which enables you to add the other person's certificate to your Contacts. You can't encrypt email messages without a digital ID. So if this is a first time deal then send a digitally signed document at first, once they respond with their own digitally signed email, then you can start encrypting.
To encrypt messages, you can use the public-key cryptographic system. In this system, each participant has two separate keys: a public encryption key and a private decryption key. When someone wants send you an encrypted message, he or she uses your public key to generate the encryption algorithm. When you receive the message, you must use your private key to decrypt it.
References:
https://www.sslsupportdesk.com/digitally-signing-encrypting-email-messages-outlook-guide/
https://support.mozilla.org/en-US/kb/digitally-signing-and-encrypting-messages