Email Received: headers often say "With ESMTPS" but this phrase seems to have no standard describing it.

is there an RFC for Protocol ESMTPS?

RFC5321 only lists SMTP and ESMTP


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.

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