Slightly OT - MX RR Santity Check requested...

[Kevin Darcy]

Barry Margolin wrote:
> In article <euf4o6$98o$1 at sf1.isc.org>,
>  Mark Andrews <Mark_Andrews at isc.org> 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