Need some help on this...

On the admin page, global settings, MTA tab, I originally checked all the following:

Protocol Checks:
Hostname in greeting violates RFC
Client must greet with a ...
Sender address must ...

DNS checks:
Client's IP Address...
Hostname in greeting...
Sender's domain...

One of my users is getting some of his emails bounced by Zimbra. These messages are not spam but I think they have to do with the security settings that I have chosen.

The message that the originator gets back is:

The e-mail system was unable to deliver the message, but did not
report a specific reason. Check the address and try again. If it still
fails, contact your system administrator.
: Helo
command rejected: Host not found>

Am I correct in assuming the originator's system is very lax in security and they are not following industry standards? Is this just a situation where they have a lazy sys admin who doesn't really know what he/she is doing?

What setting, in the Zimbra admin/global settings/MTA page do I have to undo so that this email can get through? I don't want to undo everything.

Since using Zimbra and all the special checks from the MTA tab the amount of SPAM has been almost reduced to 0.

Thanks in advance.