[Mimedefang] mimedefang-multiplexor keeps dying

[Bill Maidment]

Hi guys
I've been getting mimedefang-multiplexor dying for some time now on my external mail server with mimedefang-2.68. I don't recall it happening on 2.67.
The situation is always when filter_recipient detects an invalid username from the internal mail server.
My filter rules (mimedefang-filter) have not changed for a long while, but spamassassin and clamav rules are obviously changing frequently.
I am using Fedora 12 with sendmail-8.14.4, mimedefang-2.68, spamassassin-3.3.1-2, clamav-0.96 on the external mail server.
The external mail server log shows:

May  2 15:01:47 stiles mimedefang-multiplexor[20703]: Slave 3 stderr: panic: top_env
May  2 15:01:47 stiles mimedefang-multiplexor[20703]: Slave 3 died prematurely -- check your filter rules
May  2 15:01:47 stiles mimedefang[20718]: Error from multiplexor: ERR No response from slave
May  2 15:01:47 stiles sendmail[5978]: o4251fPE005978: Milter: to=<billnn at maidment.com.au>, reject=451 4.3.2 Please try again later
May  2 15:01:47 stiles mimedefang-multiplexor[20703]: Reap: slave 3 (pid 5277) exited normally with status 1 (SLAVE DIED UNEXPECTEDLY)
May  2 15:01:47 stiles mimedefang-multiplexor[20703]: Slave 3 resource usage: req=7, scans=0, user=0.021, sys=0.013, nswap=0, majflt=0, minflt=970, maxrss=29072, bi=0, bo=0
May  2 15:01:47 stiles sendmail[5978]: o4251fPE005978: from=<billnn at maidment.com.au>, size=12335, class=0, nrcpts=0, proto=ESMTP, daemon=MTA, relay=[117.193.171.140]

The internal mail server log shows:

May  2 15:01:47 busby sendmail[18758]: o4251lkk018758: <billnn@[192.168.2.16]>... User unknown
May  2 15:01:47 busby sendmail[18758]: o4251lkk018758: lost input channel from stiles.maidment.vu [192.168.2.12] to MTA after rcpt
May  2 15:01:47 busby sendmail[18758]: o4251lkk018758: from=<billnn at maidment.com.au>, size=0, class=0, nrcpts=0, proto=SMTP, daemon=MTA, relay=stiles.maidment.vu [192.168.2.12]

Any ideas what is causing this?
Any suggestions as to how to debug it?

Cheers

Bill Maidment
Consultant to Elgas Ltd
"It's important to keep your rough edges" - Neil Hannon