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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

7

How about RFC3848

Abstract

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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