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Exchange 2003 and number of MX Records queried

Posted on 2004-10-19
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Last Modified: 2008-03-04
We have one particular domain which has 3 MX preferences 10 15 and 20
All mail for this domain is being queued at present on my exchange 2k3 server for this domain, then after the default number of tries sends users 4.4.7 NDR

Actual bounce
donna.parker@diamond-rec.co.uk on 15/10/2004 11:05
  Could not deliver the message in the time limit specified. Please retry or contact your administrator.
  <W2K3DC01.patton.co.uk #4.4.7>

However after running an nslookup I can setup an smtp session on the MX20 preference.

I can't find a setting anywhere within exchange 2k3 that forces it to try more than 2 mx preferences.

Many Thks
BJ Magill
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Question by:BJMagill
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Sembee earned 250 total points
ID: 12351060
If that is the actual domain name, then according to dnsreport.com there is only one MX record - not three.

Exchange will only try the other MX record servers if it gets no response from the lowest server. If it gets any kind of response, and that response doesn't tell Exchange that the server is no longer responsible for that domain, then Exchange will keep trying the lowest preference server. That is by design of SMTP, not Exchange.

Simon.
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Expert Comment

by:susanzeigler
ID: 12351527
The serial number on the DNS for that domain is from today--this indicates that a change to the zone file probably happened sometimes today. Possibly they were/are in the process of changing providers or mail servers and the situation will right itself in the next day or so.
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Author Comment

by:BJMagill
ID: 12356166
That's right the hosting company made changes to DNS yesterday and mails are now being delivered, however my question still reamins.  

Before the change I could setup an smtp session with highest preference but not the two lowest which indicates that only 2 where being tried.  Is this possible.

Will Exchange SMTP session try ALL MX preferences ?
Is there a setting within Exchange to handle this ?

Thks
BJM
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by:Sembee
ID: 12357045
There is no setting that I am aware of that changes the behaviour of Exchange from what I outlined in my earlier message. Exchange will NOT try other MX records unless it gets a response that tells it that the primary MX record is no longer valid. This is by design of SMTP.

Simon.
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Assisted Solution

by:susanzeigler
susanzeigler earned 250 total points
ID: 12363730
As Simon said, it is not an issue with Exchange. Exchange was not getting the correct response from one or the other of the two lower MXs and so it either only tried one and failed or it tried two and failed. The failure did not indicate to Exchange that it should try another MX. This is an issue on the other mail server, not on yours. If no server was answering at all, then Exchange would have gone on to the next MX. Since it didn't continue to the valid one, it means that a machine answered at one or the other of  MX IPs but that completion of the send didn't happen for some reason. This is solely part of the SMTP protocol and not a failure or inability of Exchange to attempt further delivery. It is simply the system working the way it is supposed to. It is also possible that the Exchange server was looking at outdated DNS zone info that didn't have the working MX in it which resulted in failure.

If there is a persistant problem with a domain, and delivery is critical, a workaround would be to create a zone file on the DNS server that Exchange looks to for resolution, duplicating the live zone and then omit the offending MX(s). This is NOT recommended because anytime the other company changes their zone, you would need to change yours. As a temporary solution, it can work but should only be tried if you fully understand the workings of DNS and remember to remove the fake zone when all is working again.
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