This can be done in the Admin Portal, under:
I would like the Zimbra webmail app to send outgoing mail to a different SMTP server and port, which would then proxy/forward the request back to the Zimbra SMTP on port 25settings UI
Web mail MTA Hostnames
I put our ASSP ip address in instead of the hostname. ASSP sends the mail back to Zimbra's DMZ IP address after SPAM processing.
Ben Franklin quote:
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety."