4.51.5 scanning messages multiple times

[jf]

I am running MailScanner 4.51.5 (recently upgraded from 4.50.15) on 4
RedHat AS4 boxes with the same hardware.  The sendmail and MailScanner
configuration files are identical, and the load is distributed to all
four with a content switch.  Three of the boxes are working fine, but on
one of the boxes messages are being scanned several times before being
delivered.  This is happening with almost every message on that server.
 Only one message is delivered, but it is scanned many times.

The only other difference I can find between this server and the three
others is that on this one, I see lots of "[MailScanner] <defunct>"
processes, where the other servers have none.

The only semi-relevant archived suggestions I can find deal with
duplicate deliveries and the lock type.  The lock type is set to posix
on all four servers.  Changing it to flock  does not remove the behavior.

Below is a log excerpt showing this problem occurring with one message.

If anyone has any suggestions, I'd appreciate them.

--- start example log -------------------
Mar  9 04:42:30 mailforward-d sendmail[22343]: k299gIL0022343:
from=<u789mobs at maxmailer.net>, size=2312, class=0, nrcpts=1, msgid=<
200603090941.k299fL1f089152 at ns1.maxmailer.net>, proto=ESMTP, daemon=MTA,
relay=ns1.maxmailer.net [216.171.216.248]
Mar  9 04:42:30 mailforward-d sendmail[22343]: k299gIL0022343:
to=<user at emich.edu>, delay=00:00:00, mailer=esmtp, pri=32312, st
at=queued
Mar  9 04:57:46 mailforward-d MailScanner[31296]: SpamAssassin cache hit
for message k299gIL0022343
Mar  9 04:57:46 mailforward-d MailScanner[31296]: Message k299gIL0022343
from 216.171.216.248 (u789mobs at maxmailer.net) to emich.edu is spam,
SpamAssassin (score=6.005, required 5, URIBL_JP_SURBL 2.46,
URIBL_OB_SURBL 3.21, unsub13 0.33)
Mar  9 04:57:46 mailforward-d MailScanner[31296]: Spam Actions: message
k299gIL0022343 actions are deliver
Mar  9 04:57:57 mailforward-d MailScanner[31587]: SpamAssassin cache hit
for message k299gIL0022343
Mar  9 04:57:57 mailforward-d MailScanner[31587]: Message k299gIL0022343
from 216.171.216.248 (u789mobs at maxmailer.net) to emich.edu is spam,
SpamAssassin (score=6.005, required 5, URIBL_JP_SURBL 2.46,
URIBL_OB_SURBL 3.21, unsub13 0.33)

... (same thing goes on for a bit)

Mar  9 05:05:16 mailforward-d sendmail[2722]: k299gIL0022343:
to=<user at emich.edu>, delay=00:22:47, xdelay=00:00:00, mailer=esmtp,
pri=122312, relay=server.emich.edu. [serverip], dsn=2.0.0, stat=Sent (Ok.)
--- end example log -------------------