Despite what /etc/mail/README says, updating Netinfo as indicated appears to cause problems. I noticed this because sendmail couldn't find the MSP queue (/var/spool/clientmqueue) -- although it is specified in submit.cf.
The problem seems to be that Sendmail can't find submit.cf. Command-line option -Ac doesn't fix this, but if you specify the path (-C/etc/mail/submit.cf), everything is hunky-dory.
I chose to unset the Netinfo property, and then -Ac worked. Perhaps there are other Netinfo properties that sendmail is looking for. Until it's clear what those are, I'm going to leave Netinfo alone.
I found another account of this problem from Daniel J. Luke at: