Len, thank you for your deep answer. I learned a lot.
regards
-Luis Arango
> -----Original Message-----
> From: [EMAIL PROTECTED] [mailto:IMail_Forum-
> [EMAIL PROTECTED] On Behalf Of Len Conrad
> Sent: Martes, 24 de Mayo de 2005 03:30 p.m.
> To: [email protected]
> Subject: RE: [IMail Forum] How Imail handles delivery when the MX record
> has multiple IPs
>
>
> >Sandy: I did. If tests are correct, it seems like Imail doesn't lookup
> for
> >the next IP in the MX record when various IPs are registered in the MX
> >Record. Instead it goes to the next MX record.
>
> The MX algorithm says all MX records at a given preference must be tried
> before moving to the next lower preference MX.
>
> The MTA resolves an MX hostname to an A record, and tries it. The A record
> selected to be tired is usually only the first physical A record in the
> answer packet, if a record-set of A records in an an DNS answer packer.
>
> here's the example of a domain having a record-set of MX records:
>
> %dig aol.com mx
>
> ; <<>> DiG 9.2.3 <<>> aol.com mx
> ;; global options: printcmd
> ;; Got answer:
> ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 45549
> ;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 4, ADDITIONAL: 18
>
> ;; QUESTION SECTION:
> ;aol.com. IN MX
>
> ;; ANSWER SECTION:
> aol.com. 3534 IN MX 15 mailin-03.mx.aol.com.
> aol.com. 3534 IN MX 15 mailin-04.mx.aol.com.
> aol.com. 3534 IN MX 15 mailin-01.mx.aol.com.
> aol.com. 3534 IN MX 15 mailin-02.mx.aol.com.
>
> all MX records of equal value, so all MX hostnames are to be tried,
> usually
> in the physical order of the DNS answer packet above.
>
> The MX algorithm doesn't address the case of a A record-set for an MX,
> such
> as this:
>
> %dig mailin-03.mx.aol.com.
>
> ; <<>> DiG 9.2.3 <<>> mailin-03.mx.aol.com.
> ;; global options: printcmd
> ;; Got answer:
> ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18764
> ;; flags: qr rd ra; QUERY: 1, ANSWER: 4, AUTHORITY: 2, ADDITIONAL: 2
>
> ;; QUESTION SECTION:
> ;mailin-03.mx.aol.com. IN A
>
> ;; ANSWER SECTION:
> mailin-03.mx.aol.com. 288 IN A 64.12.137.249
> mailin-03.mx.aol.com. 288 IN A 64.12.138.120
> mailin-03.mx.aol.com. 288 IN A 205.188.158.121
> mailin-03.mx.aol.com. 288 IN A 64.12.137.152
>
> The order of the A records is determined by the DNS answering the query,
> NOT by AOL's DNS. The order should be considered effectvely random and
> uncontrollable.
>
> It's the MTA's implementation to decide to use only 64.12.137.249, before
> trying:
>
> mailin-04.mx.aol.com.
>
> ... or to try all the elements in the A record-set for mailin-
> 03.mx.aol.com.
>
> 1. if the A record is not deliverable, then
>
> 2. the MTA tries the "next physical" MX record (-04 above, its A record)
> at
> the same preference level
>
> or
>
> 3. tries the MX at lower preference.
>
> 4. if no MX at lower preference, the MTA defers delivery for retry
> (re-starting the MX algorithm from the top at the next retry)
>
> >I just want to confirm this is a behavior by design, a problem or just
> miss
> >configuration from my side -probably it may be fixed by changing
> something
> >in the registry-.
>
> You're worrying about something that is buried in the implementation of
> the
> MTA's MX algorithm, which itself is extremely well-known and ancient.
> There
> is NO controversy. There are certainly variations in MTA implementation
> of
> the MX algorithm, but at this point, all the well-established MTAs have
> "got it right" for all intensive porpoises.
>
> I personally have never had to get involved in an MTA's setting at this
> level in 6+ years of SMTP admin, so I'm not sure you're barking up the
> right tree, or if there is a tree.
>
>
> Len
>
>
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