If your zimbra server can reject non existence emails, just enable Mail Proxy - Options - Verify Receivers.
Or use mail filter to block/quarantine ldap filter from your zimbra server.
Greylisting is a very effective way to combat spam mails as most spam bot will only send out spams once. But it do come with it own pro and cons. Check out below link on greylisting.
Once you have enable gresylisting, it will apply to all...
In the beginning, I did enable both reject unknown client and sender in my PMG setup. Then I notice some false positive but mainly is due to the sender did not setup their DNS properly.
So now I disable both option, instead use greylisting, DNSBL and mail filter to quarantine spam/suspicious...
If you enable reject unknown client, PMG/postfix will reject any client's IP/hostname that cannot be resolved.
If you enable reject unknown sender, PMG/postfix will reject any sender/relay domain's IP/hostname that cannot be resolved.
Both 2 are optional settings, you may encounter some false...
Sorry, is reject_unknown_client_hostname
reject_unknown_client_hostname (with Postfix < 2.3: reject_unknown_client)
Reject the request when 1) the client IP address->name mapping fails, or 2) the name->address mapping fails, or 3) the name->address mapping does not match the client IP...
PMG was not design to do SMTP authentication with your email servers.
If your wordpress reside in the same network subnet with your PMG and email server, most properly it is being whitelisted as all are in the trusted network subnet.
Try Mail Proxy -> Options -> Verify Receivers
For my case, make sure you have meet the reject_nnknown_recipient_domain's requirement to work, which is missing MX record in my local DNS server.