[ On Tuesday, October 17, 2000 at 21:27:46 (-0700), Trevor Sky Garside wrote: ]
> Subject: Re: [Exim] Failing behviour based on SMTP codes.
>
> My thoughts on this would be that one should treat a 5xx response from a
> *primary* (i.e. lowest preference number) MX as permanent, but if the
> response came from one of the less preferable MX hosts, it should be treated
> as if that host was unreachable. This would be useful in this example:
>
> MX 5 mail.domain.com. (down)
> MX 10 backup.otherdomain.com. (temporarily broken)
> MX 15 never.broken.mail.host. (working great)
Ah, no, there's no such thing as "temporarily broken". Either it works,
or it doesn't. If it's knowingly "temprarily unavailable" then it'll
return a temporary SMTP reply code (eg. 421) and things will work as
expected. If it's returning 5xx errors then mail to that address is
undeliverable by definition and it must be bounced. There are no if's
and's or but's here -- the rules are pretty simple.
If your scenario cannot tolerate the possibility that a secondary MX
will go bonkers and start handing out 5xx responses when it shouldn't
then don't use it in the first place! There's almost no valid reasons
remaining in this modern world to ever use secondary MXers anyway, and
indeed many good reasons other than just this to avoid them altogether.
--
Greg A. Woods
+1 416 218-0098 VE3TCP <gwoods@???> <robohack!woods>
Planix, Inc. <woods@???>; Secrets of the Weird <woods@???>
