Fully qualified hostname problem

[Müfit Eribol]

Hi,

I have a naive question. I am managing mail server of a small company 
with two domain names.

The fully qualified name of server is xxx.xyz.com. When I send mail from 
xyz.com domain, there is no problem. But, if I send mail from abc.com 
which is being handled by the same server, we get "Helo command 
rejected: need fully-qualified hostname".

I can I have 2 domains properly co-exist on the same server so that we 
will not have this problem.

Thank you.