Received: headers often say "With ESMTPS" but this phrase seems to have no standard describing it.
is there an RFC for
RFC5321 only lists
How about RFC3848
This registers seven new mail transmission types (ESMTPA, ESMTPS, ESMTPSA, LMTP, LMTPA, LMTPS, LMTPSA) for use in the "with" clause of a Received header in an Internet message.
The new keyword "ESMTPS" indicates the use of ESMTP when STARTTLS is also successfully negotiated to provide a strong transport encryption layer.