MailScanner + Sendmail = "user unknown"

[Rich West]

I've inherited a MailScanner setup that is pretty questionable (from a
security standpoint), and I'm rebuilding the box from scratch.  I've
gotten everything installed (CentOS, clamav, SA, MailScanner, Sendmail)
to have the system act as a relay to an exchange backend.

Oddly, it does not seem to be picking up the messages that are being
left in /var/spool/mqueue.in.  I see the messages being deposited there,
but they don't seem to be acted upon.  Is there, perhaps, setting that I
might have missed/glossed over that is obvious?

-Rich