Add (optional) trace headers
According to RFC2821:
When forwarding a message into or out of the Internet environment, a gateway MUST prepend a Received: line, but it MUST NOT alter in any way a Received: line that is already in the header.
"Received:" fields of messages originating from other environments may not conform exactly to this specification. However, the most important use of Received: lines is for debugging mail faults, and this debugging can be severely hampered by well-meaning gateways that try to "fix" a Received: line. As another consequence of trace fields arising in non-SMTP environments, receiving systems MUST NOT reject mail based on the format of a trace field and SHOULD be extremely robust in the light of unexpected information or formats in those fields.
The gateway SHOULD indicate the environment and protocol in the "via" clauses of Received field(s) that it supplies.
This program definitely forwards "a message into or out of the Internet environment" so adding a trace Header is required. The format is defined in RFC2822.
However, it should be possible to disable this behaviour since depending on the environment editing the E-Mail in any way shape or form may not be desired.