How can I automatically copy messages based on sender or recipient addresses?
It would require custom programming. You could either write a mail filter using the new Milter API in sendmail 8.10 and later (see libmilter/README) or you could look at some of these other unsupported hints: • Axel Reinhold’s logall.c module • Message-Id: <33AA593A.4B701C3F@syntegra.nl> • the procmailrc man page Note that no such feature has been added to sendmail. When asked about this one of the sendmail developers said it was “because we still believe a bit in privacy.