Re: [Exim] Failing behviour based on SMTP codes.

John Sloan rearranged electrons thusly:

> Consider the case of a intermittently connected host who uses their ISP as
> backup MXes. Let us assume that that ISP may have filters on thier MX
> hosts which aren't the same as those on their customers' SMTP host (spam
> blocks most likely).

Which some admins apply to _freemail_ services like hotmail / yahoo - but
nowhere else (as lots of spam comes in from <spambag@???> either
direct to MX or through an ISP's server - or through a hijacked 3rd party open
relay.

As most of the large American ISPs have applied port 25 blocking across their
dhcp pools (where most intermittently connected hosts live), most people would
smarthost through their own ISP's smtp server, rather than relay through their
own domain's MTA, which might be remotely hosted (and ideally using some form
of AUTH for relaying).

Plus, most such dialup pools are already on the MAPS DUL - so trying direct to
MX and using an ISP's smtp as a fallback MX may still result in your mail
being rejected.

Any ISP still implementing such blocks (parsing received lines with the from
header) on a large scale would be dropping several thousand legit mails into
dev/null

> Equally well there is an argument that only the lowest value MX should be
> able to make that sort of decision.

Yes. Theoretically.

> Of these I think that the first - that an 5xx means 5xx, no matter whom it

Yes. Practically.

--
Suresh Ramasubramanian + sureshr@???
Linux is like a Teepee - No Windows, No Gates, Apache Inside