Slightly OT - MX RR Santity Check requested...

Mark Andrews Mark_Andrews at
Thu Mar 29 02:28:50 UTC 2007

> In article <euf4o6$98o$1 at>,
>  Mark Andrews <Mark_Andrews at> wrote:
> > > You mean "their" configuration is broken?  The sending mail server is NOT
> > > ours.  We're on the receiving end.  
> > 
> > 
> > 	No.  Your configuration is broken.  The lowest preference
> > 	MXs MUST always be reachable.  You cannot depend upon
> > 	fallback to higher preference MXs.  The sending side is not
> > 	required to try them.  It is required to try all the lowest
> > 	preference MXs.
> RFC 2821 seems to contradict you:
>    When the lookup succeeds, the mapping can result in a list of
>    alternative delivery addresses rather than a single address, because
>    of multiple MX records, multihoming, or both.  To provide reliable
>    mail transmission, the SMTP client MUST be able to try (and retry)
>    each of the relevant addresses in this list in order, until a
>    delivery attempt succeeds.  However, there MAY also be a configurable
>    limit on the number of alternate addresses that can be tried.  In any
>    case, the SMTP client SHOULD try at least two addresses.

	This does not cover how the list of addresses is choosen.
	RFC 952 still applies.  The list of addresses can legally
	be those of just the lowest preference MXs.

> -- 
> Barry Margolin, barmar at
> Arlington, MA
> *** PLEASE post questions in newsgroups, not directly to me ***
> *** PLEASE don't copy me on replies, I'll read them in the group ***
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark_Andrews at

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