I don't run a mailserver. That is why I just did a manual DNSBL lookup to show you what to expect in the tcpdump output.
You will have to determine using
tcpdump whether sendmail is doing DNSBL lookups while processing the incoming mail.
From the
rc.conf stuff you posted, I saw you use clamav virusscanning. This really can be a resource hog. You could check if you are not somehow running out of resources like file descriptors, memory space etc.
Another point is the
/etc/rc.d/sendmail status output complaining that "sendmail_clientmqueue" is not running.
Did you take the necessary steps to "compile' the sendmail mc file into the sendmail.cf format? And make sendmail re-read this file?
I am not really a sendmail expert. If the above suggestions don't work you may be better off to ask on a sendmail mailing list