Client host rejected: cannot find your hostname

[omegainstitute]

I'm having a problem with some of the emails coming into our Zimbra Server.

SMTP error from remote mail server after RCPT TO:<xxxxxxxx@domain.com>:
    host mail.domain.com [xxx.xxx.xxx.xxx]: 450 4.7.1 Client host rejected:
    cannot find your hostname, [66.228.124.187]: retry timeout exceeded

Under Global Settings -> MTA
I have for Protocol Checks:

(Checked)   Hostname in greeting violates RFC (reject_invalid_hostname)
(Not Checked) Client must greet with a fully qualified hostname (reject_non_fqdn_hostname)
(Checked)  	Sender address must be fully qualified (reject_non_fqdn_sender)

DNS Settings:

(Checked) Client's IP address (reject_unknown_client)
(Unchecked) Hostname in greeting (reject_unknown_hostname)
(Checked) Sender's domain (reject_unknown_sender_domain)

Now, problem is, if I were to do a reverse IP lookup for 66.228.124.187 I get
a valid DNS name back (This is done via nslookup being run on the Zimbra machine).

What am I missing here in order to get these bounces to stop happening?

[peter@mxtoolbox.com]

It looks like their server might not be giving a proper qualified helo?

Also, the forward of their hostname does not resolve to the IP that you reversed from. 66.228.124.187 -> secure01.arwebserver.com -> 208.101.16.220. I'm not sure that's the problem, but it could be.

[omegainstitute]

Just to be clear, which option should I turn off to allow these emails to come into my system? And thank you for your assistance BTW..

[ArcaneMagus]

reject_unknown_client is the option blocking this instance. A lot of mail servers are improperly configured and will be blokced by this so I leave it unchecked even though most of those servers are the spam senders, there are still valid ones that fail this test. For more information check out Postfix Configuration Parameters

[veronica]

You might also think to disable this option:-

reject_non_fqdn_hostname

reason being mail clients like outlook in many cases don't greet FQDN ( and greet with their system name ) in which case your server wont accept those mails.