EXCHANGE 3499 (000B09B6) Host unreachable

Posted on 2006-04-03
Medium Priority
Last Modified: 2009-12-16

There are certain domains that we send mail to that get rejected back to us with the message: EXCHANGE 3499 (000B09B6) Host unreachable.

After searching the Experts Exchange site and reading some of the other similar posts, I have confirmed that we are not on any blacklists and a test of reverse DNS does in fact indentify us.

Any ideas on why this would be happening?


Question by:Pepfontana
  • 2
LVL 104

Expert Comment

ID: 16361995
Host unreachable normally means what it says.
If you know what domains it is, then you could first do an nslookup on the domain from the Exchange server, to see whether you get anything, then compare it with the results from something like dnsstuff.com

Default Server:  server.domain.co.uk

> set type=mx
> microsoft.com
Server:  server.domain.co.uk

Non-authoritative answer:
microsoft.com   MX preference = 10, mail exchanger = mailc.microsoft.com
microsoft.com   MX preference = 10, mail exchanger = maila.microsoft.com
microsoft.com   MX preference = 10, mail exchanger = mailb.microsoft.com

maila.microsoft.com     internet address =
maila.microsoft.com     internet address =
mailb.microsoft.com     internet address =
mailb.microsoft.com     internet address =
mailc.microsoft.com     internet address =
mailc.microsoft.com     internet address =

When you have the MX server information, see if you can telnet to port 25 of the remote server.
For example (using the above information)

telnet maila.microsoft.com 25


Author Comment

ID: 16362057
Thanks Simon,

When I try the nslookup, it finds the server and IP address, but it also returns the message "DNS request timed out".

Thanks again!
LVL 104

Accepted Solution

Sembee earned 2000 total points
ID: 16362228
That would indicate the server is having problems with DNS lookups, which would explain the messages.

The best practise that I use for DNS is quite simple.

On the Exchange server itself, on the network card, configure the DNS servers as the domain controllers ONLY.

On the domain controllers, configure the DNS servers on the network card to be itself as primary and another domain controller as secondary (if available). If you only have one DC, then only enter one DNS server.

No external DNS servers are in the network configuration.

Then open up the DNS server Administrative Tool, and configure forwarders, using your ISPs DNS servers as forwarders.


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