My zimbra configuration is properly rejecting email from a server with the following message in the log:
Apr 29 21:09:46 zimbra postfix/smtpd[16318]: NOQUEUE: reject: RCPT from[]: 504 5.5.2 <web01>: Helo command rejected: need fully-qualified hostname; from=<theiraddress> to=<myaddress> proto=ESMTP helo=<web01>
My question is, what do I need to tell the administrator of the sending server to fix exactly?

Thanks in advance.