Here's how an email message can be reformatted or altered in transit, between a sending MTA and a recipient's MUA.

First off, almost all MTAs and forwarding agents will prepend a Received: header noting the handover details (date of reception, hostname of receiving MTA, HELO name of sending agent, and IP address and possibly rDNS of sending agent).

Mailing List Managers

Mailman, appends the headers List-Id, List-Owner, List-Post, List-Subscribe, List-Unsubscribe, List-Help, Precedence, Sender, and Reply-To, by default.

ezmlm prepends the headers Reply-To:, Precedence:, Mailing-List: . Some ezmlm lists seem to rename the old Message-Id header to X-Message-Id and create a new one.

LYRIS list software will move Message-Id to X-Message-Id and create a new Message-Id.

Yahoo! Groups adds X-Originating-Ip, X-Yahoo-Profile, Mailing-List, Delivered-To, Precedence, List-Unsubscribe, X-Sender, X-Apparently-To.

Relays

MS Exchange will reformat From, To, Received, and Message-Id headers, and may remove headers and even entire MIME parts, depending on its configuration.