Re: [exim] Re: bounce messages and their potential misuse

On 30/03/06, Adam Funk <adam00f@???> wrote:
> On 2006-03-30, Peter Bowyer <peter@???> wrote:
> > On 30/03/06, Adam Funk <adam00f@???> wrote:
> >> On 2006-03-30, Nigel Wade <nmw@???> wrote:
> >>
> >> > That only works for mis-configured MTAs. A properly configured MTA would reject
> >> > a message destined for a non-existent recipient. It would not accept it and then
> >> > generate a bounce message.
> >>
> >> But when MTA(n) rejects a message that MTA(n-1) is trying to relay,
> >> MTA(n-1) has to bounce it, right?
> >
> > MTA(n-1) shouldn't accept messages to invalid recipients in the first
> > place. If it has no direct knowledge of valid recipients, it should do
> > callouts.
>
> I understood those weren't reliable because (there may be other
> reasons?) in many cases MTA(n) is configured not to give out that
> information because spammers could use it.

The usual use case here is a 'border' MTA receiving mail for a known
list of domains and forwarding to inner mailbox servers. In those
controlled circumstances, recipient callouts are just fine. They
shouldn't be used to indiscriminate destinations - but an MTA
shouldn't be relaying for indiscriminate destinations either.

Peter



--
Peter Bowyer
Email: peter@???