psmtp is postini, which tends to know what they're doing.
It is unusual for postini not to say so forthrightly, but part of the issue might be a problem with your DNS coupled with the fact that you're not a known email server (SenderBase® The IronPort Security Network). Lots of sites will see the lack of a forward DNS entry for 74-53-208-72.site5.com as a problem. Your hosting provider should be familiar with the issue.
$ nslookup zimbra.chapmans.co.tt
$ host 18.104.22.168
22.214.171.124.in-addr.arpa domain name pointer 74-53-208-72.site5.com.
$ host 74-53-208-72.site5.com
Host 74-53-208-72.site5.com not found: 3(NXDOMAIN)