Downgraded-Final-Recipient mail standard [RFC6857 Section 3.1.10]
Downgraded-In-Reply-To mail standard [RFC6857 Section 3.1.10]
Downgraded-Message-Id mail standard [RFC6857 Section 3.1.10]
Downgraded-Original-Recipient mail standard [RFC6857 Section 3.1.10]
Downgraded-References mail standard [RFC6857 Section 3.1.10]
Some of the "Downgraded" headers are still standardized, according to http://www.iana.org/assignments/message-headers/message-headers.xhtml
Downgraded-Final-Recipient mail standard [RFC6857 Section 3.1.10] Downgraded-In-Reply-To mail standard [RFC6857 Section 3.1.10] Downgraded-Message-Id mail standard [RFC6857 Section 3.1.10] Downgraded-Original-Recipient mail standard [RFC6857 Section 3.1.10] Downgraded-References mail standard [RFC6857 Section 3.1.10]