I have a zimbra server on an ip ending in xxx.xxx.xxx.94 and when sending mail the receiving servers see the another computer's ip at the beginning of my ip block of xxx.xxx.xxx.90.

This makes me get 550's like this from some domains.
550 OU-001
Unfortunately, messages from xxx.xxx.xxx.90 weren't sent.

I also get 554's like this:
554 5.7.1 : Client host rejected: please_use_SMTP_relay_of_your_isp,_or_setup_non-generic_dns.

any help would be appreciated!!!