Body none reserved [RFC6068]
Archived-At mail standard [RFC5064]
Authentication-Results mail standard [RFC7601]
Auto-Submitted mail standard [RFC3834 section 5]
DKIM-Signature mail standard [RFC6376]
MT-Priority mail standard [RFC6758]
Organization mail informational [RFC7681]
Original-From mail standard [RFC5703]
Original-Recipient perm/original-recipient mail standard [RFC3798][RFC5337]
Original-Subject mail standard [RFC5703]
Received-SPF mail standard [RFC7208]
Require-Recipient-Valid-Since mail standard [RFC7293]
Solicitation mail [RFC3865]
VBR-Info mail standard [RFC5518]
From this list, ignoring headers listed in other issues, we have a number of miscellaneous headers defined in various RFCs. http://www.iana.org/assignments/message-headers/message-headers.xhtml
Body none reserved [RFC6068] Archived-At mail standard [RFC5064] Authentication-Results mail standard [RFC7601] Auto-Submitted mail standard [RFC3834 section 5] DKIM-Signature mail standard [RFC6376] MT-Priority mail standard [RFC6758] Organization mail informational [RFC7681] Original-From mail standard [RFC5703] Original-Recipient perm/original-recipient mail standard [RFC3798][RFC5337] Original-Subject mail standard [RFC5703] Received-SPF mail standard [RFC7208] Require-Recipient-Valid-Since mail standard [RFC7293] Solicitation mail [RFC3865] VBR-Info mail standard [RFC5518]