Quoting Chris Cameron (chris@onemind.com):

> define(`confLOCAL_MAILER', `cyrus')


This is the key part.  Anything that resolves to Mlocal, will
instead be sent to the cyrus mailer.

This really wants you to catch and alias mail for root
and postmaster.

There are rules that might be added for "CyrusExceptions"
that would truly deliver to Mlocal.  Bumps I've had with
Sendmail's commercial IMAP server (based on Cyrus) have
included that it rejects illegal mails that might contain
nulls and other cruft that should never get to you, but does.


OTOH, if all mail -> cyrus isn't desired, I've used mailertable to
send all users aliases (by hand) to say "cyrus     cyrus:localhost"
Add a "CPcyrus" to LOCAL_CONFIG  and aliases like:
chuck:   chuck@cyrus

to get my mail into cyrus.