Most other applications add the TLS Version and used cipher to the received header.
from xyz.acme.local (10.100.1.234) by abc.acme.local (10.100.109.233) with Acme SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA256_P256) id 15.1.1261.35; Tue, 17 Jul 2018 07:22:15 +0200
It would make sense that Domino adds this to the received header as well!
And maybe into a separate text list indicating the message was transport encrypted
Daniel Nashed [ https://blog.nashcom.de ]