Slightly OT - MX RR Santity Check requested...

Barry Margolin barmar at
Thu Mar 29 02:07:54 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.

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 ***

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