We see something strange on our system in regards to mail coming in via rmail from a UUCP link. The sender is being set to mailmaster instead of the real sender, and a Sender: header is being added to the message.

If mailmaster is the user that is running rmail, you need to include that user in the trusted_users configuration option. Only trusted users are permitted to specify senders when mail is passed to Exim via the command line.