What is included in an email message header?

There are over 300 fields that can contain information in an email message header. The standard mail fields include Accept, Accept-Charset, Accept-Encoding, Accept-Language, Accept-Ranges, Age, Archived-At, Authentication-Results, Auto-Submitted, Bcc, Cc, Comments, Date, DKIM-Signature, Downgraded-Final-Recipient, Downgraded-In-Reply-To, Downgraded-Message-Id, Downgraded-Original-Recipient, Downgraded-References, From, In-Reply-To, KeyWords, Message-ID, MT-Priority, Original-From, Original-Recipient, Original-Subject, Received, Received-SPF, References, Reply-To, Require-Recipient-Valid-Since, Resent-Bcc, Resent-Cc, Resent-Date, Resent-From, Resent-Message-ID, Resent-Sender, Resent-To, Return-Path, Sender, Subject, To, and VBR-Info.