MX preferences do not work

  • Thread starter Thread starter Doug Ford
  • Start date Start date
D

Doug Ford

I have setup a zone file with two MX records, one with a
preference of 10 and the other with a preference of 20. A
mail server should query DNS for all MX records for a
given domain and send the message to the server with the
lowest preference, followed by the next lowest preference
if the lowest preference server is unavailable and so on.
For some reason, messages are being sent to both the 10
and 20 preferences eventhough the server with the 10
preference is undoubtedly available 24 X 7.

Researching the problem using NSLOOKUP I noticed that a
query using 'set vc' and 'set type=MX' against the domain
returns the lower preference above the record with the
higher preference the FIRST time I query the DNS server,
but the second time I query it returns the higher
preference above the lower.

Does the order in which the records are returned have any
effect on WHICH mail server the message will be sent to?
Or are mail servers *SMART* enough to use the preference
values and ignore the order?

Regards,

Doug Ford
MCSE, MCT
(e-mail address removed)
 
In
Doug Ford said:
I have setup a zone file with two MX records, one with a
preference of 10 and the other with a preference of 20. A
mail server should query DNS for all MX records for a
given domain and send the message to the server with the
lowest preference, followed by the next lowest preference
if the lowest preference server is unavailable and so on.
For some reason, messages are being sent to both the 10
and 20 preferences eventhough the server with the 10
preference is undoubtedly available 24 X 7.

Researching the problem using NSLOOKUP I noticed that a
query using 'set vc' and 'set type=MX' against the domain
returns the lower preference above the record with the
higher preference the FIRST time I query the DNS server,
but the second time I query it returns the higher
preference above the lower.

Does the order in which the records are returned have any
effect on WHICH mail server the message will be sent to?
Or are mail servers *SMART* enough to use the preference
values and ignore the order?

Regards,

Doug Ford
MCSE, MCT
(e-mail address removed)
Click to expand...

Mail server will use the preference value, but they will cache all MX
records for a given domain.
It will attempt delivery to the server with the "10" first but if that
server is busy at that given time it will attempt delivery to the server
with the "20". Even though the first is available 24x7 it is a timing issue,
if it is busy at that time the sending server will not wait beyond its
timeout period, the only difference is SMTP servers will attemp delivery in
the order of preference. If you set preference to the same value then it
will be a random connect order.
 
The preference order is a fucntion of the SMTP server not the DNS server. DNS is this case is simply using round robin when returning the queries. It's up to
the SMTP server to then take the data and make useful meaning of it. As long as DNS is returning both records, DNS has done it's job. Order of the records in
this case, has not relevance.

Thank you,
Mike Johnston
Microsoft Network Support

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