Re: [9fans] $smtp dns failure - Plan9

This is a discussion on Re: [9fans] $smtp dns failure - Plan9 ; i'm not sure why you're taking offence. to me it seemed clear that mxlookup differs from the algorithm in section 5 of 2821 and what other mailers do. the rfc doesn't say that we need to abort of the mx ...

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Thread: Re: [9fans] $smtp dns failure

  1. Re: [9fans] $smtp dns failure

    i'm not sure why you're taking offence. to me it seemed clear that mxlookup
    differs from the algorithm in section 5 of 2821 and what other mailers do.
    the rfc doesn't say that we need to abort of the mx lookup fails. i think this
    is important because it's possible for an mx lookup to fail and an a/a6 lookup on
    the same name return results. i had quite a bit of trouble with this when using
    level-5's dns last year. in addition, a quick inspection of postfix' src/smtp/smtp_addr.c
    seems to confirm that they perform a/a6 lookup on mx lookup failure. since
    other smtp agents don't notice the misconfiguration, we are left to find it.

    anyway, i put the effort into pulling together my notes from last year on this
    to help, not to annoy. hopefull this information is not a further annoyance.

    - erik

  2. Re: [9fans] $smtp dns failure

    When talking about this, it's important to distinguish between the DNS
    reporting that there is no MX record for a domain and reporting that
    there is a transient DNS error and thus it can't tell if there is an
    MX record for a domain.

    If there is no MX record, smtp should use an A or A6 record, if it
    exists. If there is a DNS error looking up the MX, smtp should stop
    and not use any A or A6 record, since that may be the wrong
    destination, and you can't tell until the transient DNS error passes.


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