Multiple Domain in Hmail Server

We have hosting HMail Server in our company which using one public IP to hosting multiple domain name.But last week my client feedback they have facing the issue to send out email with the error below for one of the particular domain, please advice

Remote server replied: 550 Access denied - Invalid HELO name (See RFC2821 4.1.1.1)
YaYangTeahAsked:
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BudDurlandDirector of ITCommented:
Usually, this is an anti-spam measure by the receiving server.  The mail is from 'John@THIS-Domain.com', the server identifies itself as 'mail.THAT-domain.com'.  The fix is often to set the MX record for 'THIS-Domain.com' to be 'mail.THAT-domain.com'.  You should also make sure you have proper reverse DNS in place, and an SPF record can't hurt.
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YaYangTeahAuthor Commented:
Due to the we use one IP for multiple domain ,when we do the reverse DNS check found that everytime is return back to us different domain name. Please advise how to make it ,if we use the user@abc.com the reverse dns will be mail.abc.com and make sure the reverse dns return the correct result

I believe that is the reason ,can not send through also.
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BudDurlandDirector of ITCommented:
I begin to think you may have 2 problems here.  There should be one reverse DNS entry for your mail server's IP address.  It should resolve to the fully qualified host name that the mail server uses when it greets other mail servers.  Usually, this is some variation of 'mail.mydomain.com'.  for mail servers, it is important that the forward and reverse DNS entries match.  If mail.mydomain.com resolves to 10.10.10.10, the reverse DNS lookup (also known as a PTR record) for 10.10.10.10 should return 'mail.mydomain.com.  If you are controlling your external DNS, you need to correct that problem.  If your ISP is doing it, you need to work with them to fix it.

You even when sending for 'user@abc.com', your server will greet the other server as 'mail.mydomain.com'.  It can only use one name.  This is perfectly fine.   Although it's conceivable that a server would reject an incoming message because of the mis-matched domains, most will not.  Instead, they will use other mechanisms to verify the sender, and the one most often used is the MX record for the sender's domain.   As long as the MX record for the domain 'abc.com' has 'mail.mydomain.com' defined as its host, the mail should be accepted.  Something like this, I think, may be a contributing factor to your problem.
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