[Opencomputing-openprotect] op qmail kernel out of memory - big crash

[Darren Fulton - Concept Technology]

Hello Everyone,
A mailscanner server that I look after crashed on the 24th and had to 
have the button pushed to restart.  Redhat 9, 2.4.20-30.9smp #1 SMP, 
Openprotect 5.0.1.5, qmail 1.3 with vpopmail, etc.

From /var/log/messages:
Apr 18 05:05:24 mail kernel: Out of Memory: Killed process 8578 
(MailScanner).
Apr 18 22:53:06 mail kernel: Out of Memory: Killed process 24837 
(MailScanner).
Apr 22 07:37:05 mail kernel: Out of Memory: Killed process 24440 
(MailScanner).
Apr 22 11:20:24 mail qmailq: Process did not exit cleanly, returned 0 
with signal 9
Apr 24 03:57:06 mail kernel: Out of Memory: Killed process 1498 
(MailScanner).
Apr 24 03:57:08 mail qmailq: Process did not exit cleanly, returned 0 
with signal 9

Nothing interesting looking in /var/log/maillog.
Output of "free" from today:
[EMAIL PROTECTED] root]# free
            total       used       free     shared    buffers     cached
Mem:        513108     506588       6520          0       4232      11404
-/+ buffers/cache:     490952      22156
Swap:      1052216     842672     209544
Any ideas?  How can I make it exit cleanly? 

Thanks,
Darren