I was having some bad issues with spam. It was recommended that I check the following boxes in zimbraAdmin:

reject_unknown_client
reject_unknown_hostname
reject_unknown_sender_domain

I also added RBLs for:

zen.spamhaus.org
dnsbl.sorbs.net
b.barracudacentral.org

Spam stopped being a problem, BUT now we've little by little started bouncing valid vendors emails as "undeliverable"

This is a HUGE problem. So far its only about a dozen addresses, but seems to be growing quickly (only 6 last week).

using one of the problem email addresses ( _@crosbybrownlie.com), I've grep'd through logs and found:

/var/log/zimbra.log

Jan 8 09:10:15 mail postfix/smtpd[9804]: NOQUEUE: reject: RCPT from host-69-95-11-35.roc.choiceone.net[69.95.11.35]: 450 4.7.1 <CBSERVER.CBDOMAIN.local>: Helo command rejected: Host not found; from=<_@crosbybrownlie.com> to=<_@vpsupply.com> proto=ESMTP helo=<CBSERVER.CBDOMAIN.local>

Which appears several times for each attempt they've made to try again.

I've added the following rule to the /opt/zimbra/conf/salocal.cf.in:

whitelist_from *@crosbybrownlie.com

But it did not seem to affect the issue.

Is there a way to remedy this without turning off the reject rules? Hopefully one that does not require adding each domain to a safe list as I'm sure this problem will be reoccurring as time goes forward.

I need to get this figured out immediately, very large bids are being held up and we are risking loosing large accounts over it.

Thanks in advance for any help!