Re: [exim4] Testing and making sense of smtp output
On Sun, 19 Oct 2014 16:37:13 +0100
Brian <ad44@cityscape.co.uk> wrote:
>
> primary_hostname is used as the HELO but Debian doesn't set it. Also,
> the exim maintainers aren't very keen on your using it in a
> configuration file.
>
>
Many mail servers have a public IP address, and it makes sense to use
the machine's own hostname in the HELO. If the mail server is behind
NAT, it is not reachable by name, and doesn't even have an inherent
FQDN, so there is no point in using its real hostname. Indeed, unless
a public A record is created to match such a HELO, many mail servers
will refuse mail from the machine.
A made-up hostname which resolves to the public-IP side of the NAT is
then appropriate for the HELO, and will clearly not be found anywhere
in the mail server's system files. Also, if a server handles multiple
domains, it may be felt appropriate to use a separate HELO for each
domain.
--
Joe
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