Dan, you're simply wrong. Header fields have been designed that
modify essentially every aspect of email operation
No. New header fields are OPTIONAL protocol extensions.
Even "OPTIONAL" protocol extensions cause the implementations that
support those extensions to behave differently. In many cases,
such extensions have been harmful to recipients, or to operation
of the mail service.
In general, the more extensions that are in use, the more behavior
differs from one UA to another, and the less predictable and
interoperable the mail system becomes.
Keith