"To decrypt the emails, he first manipulates their ciphertext by using appropriate malleability gadgets." - so if you use triple wrapping as per https://www.iet

"To decrypt the emails, he first manipulates their ciphertext by using appropriate malleability gadgets." - so if you use triple wrapping as per https://www.ietf.org/rfc/rfc2634.txt https://www.ietf.org/rfc/rfc2634.txt you are safe. e: To make the claim more precise: you must drop messages which are not triple wrapped and those which are triple wrapped, but inner signer is different from the outer signer.