MX record error

Hi, Here is another issue I am having with my mail server. My domain's mail server is working, but another user's mail server on my Xserve is generating the following error in DNSReport: Warn: Mail server in host name greeting WARNING: One or more of your mailservers may be claiming to be a host other than what it really is (the SMTP greeting should be a 3-digit code, followed by a space or a dash, then the host name). This probably won't cause any harm, but may be a technical violation of RFC821 4.3. mail.otheruserdomain.com claims to be host mydomain.com. How can I fix this? Thanks in advance, Cory