An e-mail gateway could handle the user’s PKI asymmetric keys, perform the desired service before encrypting or decrypting e-mails. Djigzo, author of open source encryption gateway, sent in his solution. The gateway will automatically decrypt incoming messages being sent to internal users. Then the unencrypted message can be scanned, archived, or checked for data leaks, or retrieved for later retrieval. The key question is, how would the S/MIME client and encryption gateway interact?”]
Source: https://www.csoonline.com/article/2629197/s-mime-gateways-can-create-fatal-security-breakdowns.html