Using NSLookup to determine an SMTP receiving system

Background
NSLookup is a great tool that comes with Windows that allows you to search DNS for information. &nbspIt is especially useful to troubleshoot particular issues with Exchange. &nbspExchange is reliant upon DNS to know where to send outbound messages. &nbspWhen Exchange has problems getting messages to a particular domain, it's time to open the toolbox.

The best place to run NSLookup is on the server which sends out your SMTP traffic. &nbspThis will show you the same information that your SMTP engine uses when determining where to send mail to a particular domain.

Open a command prompt window
At the prompt, type nslookup
Type the command set type=mx
Type the registered domain name (e.g. domain.com)

You will receive a response similar to:

Non-authoritative answer:
domain.com MX preference = 10, mail exchanger = mail1.domain.com
domain.com MX preference = 20, mail exchanger = mail2.domain.com
domain.com MX preference = 30, mail exchanger = mail3.domain.com

Interpreting the NSLookup results
Your SMTP engine will attempt to use the MX records in ascending order according to their value. &nbspThe name associated with the MX record is what your engine will use. &nbspYou can simulate what the engine does by using the Telnet command. &nbspIn other words, the FQDN associated with the lowest numbered MX value would be the one that your SMTP engine would attempt to connect with.

Using the NSLookup results to test connectivity
In the simulated response shown above, you can test the readiness for receiving SMTP communications by using the Telnet command. &nbspIn a command-prompt window, type telnet mail1.domain.com 25. &nbspIf the system connected to the FQDN is accepting SMTP communications, you’ll receive a response.

DNS Root Servers

What are the root servers? These are the DNS servers from which all others get information.

Why would anyone care? To verify the information other mail systems see when trying to reach your SMTP domain.

How do I use the information? You can force NSLookup to poll a particular DNS server by using the command:
server ip_address or server fqdn

As an example, open a command prompt window
At the prompt, type nslookup
Type the command set type = mx
Type the command server a.root-servers.net
Type the registered domain name (e.g. domain.com)

You have requested the MX information for domain.com directly from one of the DNS root servers.

Here is IP information for the thirteen (13) root servers.
a.root-servers.net   198.41.0.4
b.root-servers.net   192.228.79.201
c.root-servers.net   192.33.4.12
d.root-servers.net   128.8.10.90
e.root-servers.net   192.203.230.10
f.root-servers.net   192.5.5.241
g.root-servers.net   192.112.36.4
h.root-servers.net   128.63.2.53
i.root-servers.net   192.36.148.17
j.root-servers.net   192.58.128.30
k.root-servers.net   193.0.14.129
l.root-servers.net   198.32.64.12
m.root-servers.net   202.12.27.33

The complete information can be found at the website: http://www.root-servers.org