I've been running some more tests (sending about 1msg per 1 or 2 seconds) and have been seeing the occasional :

Code:
Subject: ***UNCHECKED***

WARNING: possible mail bomb, NOT CHECKED FOR VIRUSES:
  Maximum number of files (1500) exceeded at /opt/zimbra/amavisd/sbin/amavisd line 5042.
I can see that I can increase the open files limit - but should I really be hitting it with sending mail to 5 test users like this? Is there a more recommended level for a zimbra install?

My ulimits are currently the CentOS4.2 defaults :
Code:
ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
file size               (blocks, -f) unlimited
pending signals                 (-i) 1024
max locked memory       (kbytes, -l) 32
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
stack size              (kbytes, -s) 10240
cpu time               (seconds, -t) unlimited
max user processes              (-u) 8167
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited