Fetchmail/Sendmail rejects

[Malcolm Kay]

On Sunday 15 August 2004 10:40, Chuck Swiger wrote:
> Malcolm Kay wrote:
> [ ... ]
>
> > Thanks guys for the responses -- this really looks the way to go.
> >
> > Does anyone know how sendmail distinguishes between:
> >  reject=451 4.1.8 Domain of sender address ................ does not
> > resolve reject=553 5.1.8 .......... Domain of sender address
> > .............. does not exist It seems the former is to be interpreted as
> > a 'temporary' condition while the latter is to be interpreted as
> > 'permanent' (and is by default deleted by fetchmail)?
>
> Sendmail pays attention to the return value from doing DNS queries.  If
> sendmail receives an NXDOMAIN response, it treats that as a permanent, 5xx
> failure code.  If sendmail gets a timeout/TRY_AGAIN, it will return a 4xx
> temp failure.
>

This sort of takes us back one more level -- how does the DNS service decide 
between responding with NXDOMAIN and a timeout/TRY_AGAIN? And does the
difference have any real significance?

> It's not clear to me why this would matter if your ISP is the one running
> the mailserver: they aren't accepting the message in either case, which
> ought to mean that fetchmail will never see it.

None of it is particularly clear to me -- but apparently my ISP's server is
not rejecting these messages.

If all mail servers rejected these messages it would seem to me to make the 
spammers endeavours rather pointless.

Thanks,

Malcolm