We have over 100k of spam emails send from our Zimbra. It appears that our Zimbra is acting as a relay for external domain users. What settings can we use to only allow domains hosted in Zimbra to send out emails?
Also, many valid emails get blocked when I enable Protocol and DNS check. Which of the 6 checks are recommended that will not affect valid emails?
- Hostname in greeting violates RFC (reject_invalid_hostname)
- Client must greet with a fully qualified hostname (reject_non_fqdn_hostname)
- Sender address must be fully qualified (reject_non_fqdn_sender)
- Client's IP address (reject_unknown_client)
- Hostname in greeting (reject_unknown_hostname)
- Sender's domain (reject_unknown_sender_domain)