DNS Settings

DNS server settings should be automatically assigned to your router, as the best DNS servers depend on the type and location of your broadband termination point. If you want to specify the DNS server settings manually then please login first automatically (don't specify DNS servers in your router) and note the DNS servers assigned to your router; feel free to then manually specify these in your router.
For more info about Microsoft DNS (Should we use Root Hints or External Forwarders) see http://support.microsoft.com/kb/323380
For more into about Microsoft Exchange SMTP (Should we use External DNS) see http://technet.microsoft.com/en-us/library/bb124221.aspx
Running Exchange 2007? Be sure you understand 'DNS sensitivity'; see http://technet.microsoft.com/en-us/library/bb676467.aspx

*** Please note that the DNS servers are dynamic and may change without notice. ***

Testing
In order to diagnose a DNS issue tests must be performed to ascertain if it's the DNS server on our network that's the problem, or the access to it, or some other issue. Bear in mind that with a Windows Server the Exchange System Manager may have external DNS specified in the "Default SMTP server" properties AND also the Windows DNS server "forward lookup" properties; you'll need to check if any of these are specified.

From a computer connected to the ADSL router:
(Run a command prompt)

ipconfig /flushdns [press Enter]
nslookup [press Enter]
(Make a note of IP address of "Default server Address" here:     ( ___________________ )

www.cnn.com [press Enter]
If you get a "Nonexistent domain" error message then DNS if failing and you'll need to follow the next steps, otherwise DNS is working; try some more hostnames if you want to be more thorough.

server 208.67.222.222 [press Enter]
www.cnn.com [press Enter]
If you now get a result then the "openDNS" servers are responding, but your DNS server (noted above) is not responding. This could be (a)the wrong DNS server or (b) device such as your ADSL router that's not responding to DNS queries.

 

Microsoft Exchange specific testing

Please download the "SMTPDiag" tool from http://www.microsoft.com/downloads/details.aspx?familyid=BC1881C7-925D-4A29-BD42-71E8563C80A9&displaylang=en
Once downloaded and saved to the computer (for example at C:\SMTPDiag) open a command prompt and run the following:

smtpdiag "administrator@mydomain.com" support@known_remote_address.com -d 1.2.3.4 /v

(Replace the 1st email address with a valid local email address of the Exchange server. Replace the 2nd email address with a known remote email address. Replace 1.2.3.4 with the external DNS server IP that is assigned to your Internet connection.)
Feel free to Copy & Paste the output results in a ticket to 123vISP Support and we will offer our comments...

 We'll need the following info:

the full results of the SMTPDiag command

the full results of an ipconfig /all command

the full results of a tracert command to the IP of the external DNS server (tracert 213.133.152.2)

the version of Exchange Server (Exchange System Manager > Servers > [server version column])

the version of Windows Server (Start > Run > winver)

the full results of an nslookup to the external email domain using the external DNS IP (nslookup ssl128bit.net 213.133.152.2)

 

For more detailed testing please log a Support ticket so that we can tailor our advice to your specific network environment.