[Exim] RBL reject, format of error message

When exim refuses a mail due to an entry in (one of) the RBLs the error message
shows up with "Subject: Returned mail: User unknown" which is not correct.
The user is well known, the delivery is not done due to the "real" reason given
in the "Transcript of session" which the "normal" user doesn't read/understand
in detail.

Cound the subject be made more precise in this case and the text placed in the
error mail be changed in a way that the "normal" user can understand it ?

-Frank Elsner (TUBerlin Postmaster)