The "HeaderWrittenBy" and "SendingSoftware" Mail Header Fields

This document proposes two new, optional mail header fields. * "HeaderWrittenBy" is designed to provide a simple attestation of the host that injected a message's primary author-level headers (such as "From") into the mail system. * "SendingSoftware" is designed to declare the Mail User Agent (MUA) that composed the message. These are intended to provide additional, simple-to-parse signals for mail filtering and analysis, to be used in conjunction with existing authentication mechanisms like DKIM.