We have implemented a policy in our network to force users who aren't using our smtp account to go though our relay server to pass email of another smtp. Our IPs were frequently getting blocked in DNSBL lists due to heavy spamming so, we need to stop our users from connecting any smtp and send millions of SPAM. If they aren't hapy to use our company's smtp account and want to use their own smtp then they have to relay the email from our relay server so we can discard SPAM actions and help imrpove our PUBLIC IPs reputation on web.
Refer me to any manual on how to
* Restrict Zimbra to accept sending message from ONLY my defined IP subnet.
* Let anyone from my subnet relay a message without any authentication.
* Define custom spam rules.