the feature I am after is similar to the postfix's relay_recipients, but I need to do it in sendmail. I have a list of known good recipient addresses on a server for which this machine acts as the net-facing frontend (mail comes in from the outside, it is pre-processed on the outer box and eventually relayed to the inner box . the corporate mail server) I cannot do call-forward or LDAP queries tho'.
It is probably a feature of one of of sendmail's *.db files, but my memory or understanding - or both - are currently failing me.
Note: This is a mailwatch/mailscanner box, so mail is queued locally, filtered, then delivered.