Can the host access centos.myvnc.com (the public IP) on port 25? Does that forward back to the host on port 25? Try telnetting from the zimbra host to the public IP on port 25, what happens?
Does the account exist in the system? I assume your domain is centos.myvnc.com. What do you get from
zmprov ga
eperez@centos.myvnc.com | grep MailTransport
If that's lmtp://centos.myvnc.com:7025
make sure that you can get from the zimbra server to the public IP on port 7025, and it gets forwarded back to the zimbra server.
WHat do you get from:
zmprov gs servername | grep zimbraSmtpHostname