I have this same thing happening; how does this work when hosting more than one domain?
For example, right now I have 123.com and abc.com.
If I run a check on abc.com, it might come back saying "WARNING: One or more of your mailservers is claiming to be a host other than what it really is...mail.abc.com claims to be non-existent host GW.123.com: 220 GW.123.com ESMTP Postfix (Ubuntu)"
/etc/mailname says it is GW.123.com.
Is the problem that it is "claiming" to be GW.123.com or is it that GW.123.com doesn't have a DNS entry? (Or both??)
I don't understand how having more than one domain affects this.