[TriLUG] email / DNS question

Alan Porter porter at trilug.org
Thu Jun 22 16:18:26 EDT 2006

At work, we have several machines that need to send email
to sysadmins, employees or customers.  Recently, we are
seeing some of these emails bounce because the sending
machine name is not resolvable on the external internet.

All of our machines are NAT-ed behind a single Linux box
that runs Shorewall and dnsmasq.  We pointed "nc.tmio.net"
to our external IP.

The gateway machine is called "ncgw.nc.tmio.net" and all
of our other machines in the building are "xxxx.nc.tmio.net".

I am a little bit confused as to whether "nc.tmio.net" is a
host name or a subdomain name.  We seem to be using it as

If I try to send mail from any of our machines, some recipients
will get the mail and others will not, depending on whether or
not their mail server checks for valid sender addresses.
Ironically, our corporate email provider is one that rejects
these emails.

I would like for the outgoing emails to keep their original
machine names in their FROM addresses.  For example, I would
like to get sysadmin emails from "root at mickey.nc.tmio.net",
and not just from "root at nc.tmio.net".

Is this a DNS issue?  Or is this something that I can configure
in postfix on the sending machines?  Or is there a deeper


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