Citation:
"The use of S/MIME encryption and MPE is also possible without any problems. This means that an email already encrypted with MPE can also be encrypted on the basis of S/MIME with the help of NoSpamProxy. In such cases, NoSpamProxy leaves the structure of the MPE-encrypted content untouched and applies the desired S/MIME encryption. The recipient must then first perform the S/MIME decryption (assuming appropriate certificates) in order to be able to fully decrypt the email."
Source: https://www.nospamproxy.de/en/combining-microsoft-purview-message-encryption-and-s-mime-it-works/
