Sendmail cannot send to own domain

[jason at dixongroup.net]

> Hello,
>
> We have many RedHat 8.0 machines here, all of which are running
> sendmail-8.12.8-9.80.
>
> All machines can use their local sendmail to email their local users.
>
> Also, all machines can use their local sendmail to email any address on
> the
> internet.
>
> However... None of the machines can use their local sendmail to email any
> address on the same domain as the machine itself, for example:
>
> The machines are called:
>
>   host1.sub.domain.com
>   host2.sub.domain.com
>   host3.sub.domain.com
>
> Each machine can happily email hobbs at mongeese.co.uk, for example, and
> each machine can email username at localhost. None of the machines can email
> username at sub.domain.com though.
>
> How can this be setup, and why is it happening? The MX record for
> sub.domain.com points to our rather old Cobalt Raq mail server.
>
> Can anyone suggest a way to get sendmail to work properly for our own
> domain
> as well??

I'm not a sendmail person (I prefer Postfix), but the problem and answer
are both fairly obvious if you think about it for a second.  Your users
are all local to the machine they read their email on.  I'm betting that
"bob at host1.sub.domain.com" could send mail to
"suzie at host2.sub.domain.com", since they explicitly specify their
recipient.

However, you're assuming too much when you drop the hostname from the fqdn
(fully-qualified domain name), and your primary MX (the Raq) doesn't know
how to pass on delivery.  If you check the maillog on that server, you'll
probably see a bunch of delivery failures.

The easiest method would be to add aliases on the Raq for each user in the
subdomain to point to the server they wish to receive mail on.

HTH.

--
Jason Dixon
DixonGroup Consulting
http://www.dixongroup.net