Slightly OT - MX RR Santity Check requested...

Kevin Darcy kcd at
Thu Mar 29 02:39:22 UTC 2007

Barry Margolin wrote:
> 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.
The MUST of the "be able to try" is effectively overridden by the MAY of 
the "configurable limit" (since there's nothing to prohibit a configured 
limit of 0). Which leaves only the SHOULD, which doesn't create a 
mandate. An implementation can try only a single address and still be 
compliant (minimally) with this text. Blame the RFC authors.

               - Kevin

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