Why does S/MIME encryption requires the sender to have a certificate of his own while only the recipient's public key is used?

S/MIME encryption uses the recipients public key to encrypt an email. Upon receiving the encrypted message the recipient can decrypt it using the private key associated with the public key that was used for encryption.

Why then does the sender need to have public and private key for his/her email in order to send an encrypted message? At least macOS Monterey does not show the options for singing and encrypting with S/MIME unless I select my email address for sending for which I have a certificate in my keychain.

Solution 1:

It seems this is necessary to make the message readable for the sender in the "Sent box" while still keeping the message encrypted. The system stores the encrypted sent message locally by using the sender's public key and sends it using the recipient's public key. I deleted my own private key from the keychain and indeed the encrypted test message in my sent box became unreadable. Ingenious!