I am wondering if when an email is forward does it lose its original headers?
Forwarding, in a mail client (MUA), usually means sending a new mail, with the original mail included in the body or as an attachment. Depending on the client, the headers of the original mail may be included verbatim (e.g., mutt) or only in a highly abridged fashion (e.g., outlook).
Some MUAs offer a remail command (it could be called something else such as resend or bounce, or be an option to the forward command). This resends the mail with exactly the same headers as when it was delivered. Delivery of a mail can change some headers (e.g., virus/spam scanners recording the result of their analysis), so the second recipient may not see exactly the same headers as the first.
Forwarding, in a mail server (MTA), means that the mail is resent to another system for delivery there. MTAs normally add a
Received: header at the very beginning when they receive a mail, so each forwarding server adds its own mark. Forwarding normally doesn't affect existing headers, unless the MTA has been specifically directed to rewrite some headers.
There are a few headers that are more likely than others to be rewritten by an MTA on forward. For example, some MTAs will rewrite
X-Envelope-… headers to match the envelope they see instead of the envelope the previous MTA (that added the
X-Envelope-… header) saw. More and more MTAs are configured to add some headers for spam and spoofing prevention, and they may throw away any existing header with the same name; there is a lot of variety in those.
On how it should be:
Forwarded messages must be recorded by filters as forwarded and then increase the authenticity of message (or decrease the various scores as seen in icloud headers) or decrease if the message is fake news.
The recording should be done by entering a new header called 'forward' or leaving some of the headers intact as in original mail so that it can be detected that a chain of message is going on.
For the sake of sender who saves himself from several clicks by composing a new message and opts from a 'sent' message to re-compose, message-id header must be changed/unique else all the forwarded/newly composed messages will be marked as spam owing to same message id header.