Exchange - DNS and SMTP relay agents

Posted on 1998-03-28
Last Modified: 2013-12-23
   I am a little lost on my conceptual understanding of what role the DNS and SMTP relay agents play in delivering the mail in exchange.  Can anybody give me an explanation perhaps with an example so even I can understand it.
Question by:jimmyr
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 2
  • 2
LVL 37

Expert Comment

ID: 1571635
DNS resolve mapping between domain name to IP address, for example, resolve the name to

SMTP realy agent acts like a router, send the message from its client to the next step to the destination, you may check your Internet mail header to help you understant this.

Author Comment

ID: 1571636
But what is a DNS relay agent? does that not exist?
LVL 12

Accepted Solution

Otta earned 60 total points
ID: 1571637
> I am a little lost on my conceptual
> understanding of what role the DNS and
> SMTP relay agents play in delivering the
> mail in exchange. Can anybody give
> me an explanation perhaps with an
> example so even I can understand it.

After you write a letter, you stuff it
into an envelope.
On the envelope, you write
and then you drop the envelope into
a Postal Service Mail Drop-Off Box,
and you've completing "relaying" your mail.
You rely on some Postal Service worker
to remove your letter from the Box,
and take it to a "processing centre",
where the routing of your message
will be determined.

An SMTP-server is the electronic-equivalent
of the worker and the processing centre.

At the processing-centre,
there are several steps.
First, a worker puts on his/her 'SMTP' hat,
and shouts "who's the top-guru
for addressing in the '.GOV' domain?".
When that top-guru answers,
the same worker asks that top-guru
"which of your assistant-gurus handles
addresses in the 'WHITEHOUSE.GOV' domain?".
When that top-guru answers the question,
then the assistant-guru is asked
"which host in your domain is designated
to receive E-mail for 'WHITEHOUSE.GOV'?".
When the answer is received,
then the assistant-guru is asked
"what's the IP-address for that host?".

Now, the worker opens a TCP/IP connection
to that IP-address, and says "hello".
When an answer ("OK") is received,
the worker says I have mail from "jimmyr".
When an answer ("OK") is received,
the worker says I have mail for 'PRESIDENT@WHITEHOUSE.GOV'.
When an answer ("OK") is received,
the worker says "here is the message",
and sends it.
When an answer ("OK") is received,
the worker says "quit".
When an answer ("OK") is received,
the worker hangs-up the connection.

This process of asking questions is the
DNS (Domain Name Server) service, i.e.,
asking for 'NS' (name-server) and 'MX'
(mail-exchanger) records.

When the IP-address is determined,
then the SMTP (Simple Mail Transport Protocol)
is used to relay the message.
Get your Conversational Ransomware Defense e‑book

This e-book gives you an insight into the ransomware threat and reviews the fundamentals of top-notch ransomware preparedness and recovery. To help you protect yourself and your organization. The initial infection may be inevitable, so the best protection is to be fully prepared.

LVL 37

Expert Comment

ID: 1571638
Jimmyr, if one DNS request can not be resolved at one DNS server, the server would forward the request to its upstream DNS server, util the request answered or no such domain name found. Such a server is a DNS relay agent.

Otta, you are good at to explain concepts from a basic step. :-))

Author Comment

ID: 1571639
I truly enjoyed your answer so I happily gave you all the points. But I have to admit bbao was the one who answered my last question.
LVL 12

Expert Comment

ID: 1571640
unfortunately, BBAO's answer is not correct.

If a DNS-server can give an "authoritative"
answer to a DNS-request, it gives it.
For example, if your computer is a member
of the '' domain, and you ask
the DNS-server in that domain for the IP-address of another host in that domain,
then it gives the answer, and no other
DNS-server is consulted.

However, if you ask the DNS-server for the
IP-address of a host "outside" of the domain,
then it does not go "upstream" to find
the answer.  Instead, it goes to the "top"
of the Internet, and asks the "root" servers
for assistance, and works "downstream".

For example, if you ask for the IP-address
then the "root" servers are asked for a
"referral" to the "authoritative" servers
for the '.COM' domain.  One of those DNS-servers
is then asked for a "referral" to the
"authoritative" DNS-servers for the 'DIGITAL.COM' domain.
Then, that DNS-server is asked the "original" question.
The reply may either be the IP-address,
or the message "don't ask me, ask the
authoritative DNS-servers for the 'ALTAVISTA.DIGITAL.COM' domain".

So, it's a "downstream" approach,
not an "upstream" approach.

Author Comment

ID: 1571641
Fair enough

Featured Post

Free learning courses: Active Directory Deep Dive

Get a firm grasp on your IT environment when you learn Active Directory best practices with Veeam! Watch all, or choose any amount, of this three-part webinar series to improve your skills. From the basics to virtualization and backup, we got you covered.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

FIPS stands for the Federal Information Processing Standardisation and FIPS 140-2 is a collection of standards that are generically associated with hardware and software cryptography. In most cases, people can refer to this as the method of encrypti…
Many of us in IT utilize a combination of roaming profiles and folder redirection to ensure user information carries over from one workstation to another; in my environment, it was to enable virtualization without needing a separate desktop for each…
Michael from AdRem Software explains how to view the most utilized and worst performing nodes in your network, by accessing the Top Charts view in NetCrunch network monitor ( Top Charts is a view in which you can set seve…
Monitoring a network: why having a policy is the best policy? Michael Kulchisky, MCSE, MCSA, MCP, VTSP, VSP, CCSP outlines the enormous benefits of having a policy-based approach when monitoring medium and large networks. Software utilized in this v…
Suggested Courses

626 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question