Which of the following actions MUST be performed when using secure multipurpose internet mail Extension (S/MIME) before sending an encrypted message to a recipient?

Digitally signing a message is a separate S/MIME operation that authenticates the sender using the sender's own private key, and it is not a prerequisite for encrypting a message to a recipient.

Obtaining the recipient's private key would compromise their security entirely; private keys must remain secret and are never shared - only the recipient's public key (via their certificate) is needed by the sender.

In S/MIME, messages are encrypted using the recipient's public key, which is embedded in their digital certificate. The sender must obtain this certificate beforehand so the encryption algorithm can lock the message in a way that only the recipient's corresponding private key can unlock. Without the recipient's certificate, the sender has no way to perform asymmetric encryption destined for that specific recipient.

Encrypting attachments separately is not a required step in S/MIME; when a message is encrypted with S/MIME, the entire message including any attachments is encrypted as part of the same operation.