Exchange server and Internet

I would like to know about the following topics.

SMTP gateway: is this a regular windows server with SMTP service enabled(started).
is SMTP gateway used only if a company has more than one ISP provider ?

in which case do we need Smart host?

DNS forwarder: I have seen some environment where there is no DNS forwarder configured. how do they send email to internet users?


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an smtp gateway server is a server that's has the MX DNS Record, and routes the mail into your company and outside your company vai smtp. In Exchange 2010 the Edge Transport Server is role for smtp gateway service. For security reasons the server shouldn't be part of your domain. This Server normally has a spam filter and behaps a virus filter, is can be a third party device like Borderware or something else. The Edge Transport Server is first Microsoft smtp gateway service from Microsoft that could be talled smtp gateway, but it's not the best you could get in my opinion.  

But at all, it is an smtp server with an MX Record to get and send smtp mail.

SMTP gateway is used to route the mail to another smtp server as smtp is one way communication.

DNS Forwarder-

This is the standard for sending mail. When Exchange needs to send mail to another domain it will look for the MX records of that domain and will attempt to contact the mailserver directly.


In this case Exchange takes your outgoing mail and sends it to another mailserver (which is called a “smarhost”, hence the name). The smarthost will deliver your mail to the other mailservers on your behalf. This is exactly what you do when you use Outlook Express to send mail using your ISP SMTP servers.

The example is Postini which filters as well as control span & send mail on behalf of you.

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SMTP Gateway : a server which is responsible for sending mail out to the Internet (next hop)
DNS forwarder : In case, if you local dns server is unable to resolve your query, IP in DSN forwarder tab, should be queried
Smarthost : a dedicate server, inside/outside domain which your exchange server is sending mail
                      smarthost is configured in the Send connector (E2k10, E2k7) and SMTP                  connector /Default SMTP VS
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jskfanAuthor Commented:
not really clear:
 you are saying that smtp gateway forward email to internet and not the other way around.
-can that be a regular windows server with SMTP services enabled?
-is it commonly placed in the DMZ?
-is every exchange server in the local LAN has the Default gateway IP address pointing to the SMTP gateway IP address?

DNS forwarder:
as I said if DNS forwarder is empty, how does the email go outside?

Smart host:
is this the ISP or domain provider where the domain and MX records are registered?
is the smart host kind of a relay? I mean email goes from exchange server to SMTP gateway to smart host then to the external recipient
I don't want to retype so posting as link.

If DNS forwarder is empty, it will check your dns & query forwarder into dns & root hints on your dns server to route the mail to other domain which is long way.

Mx records are registered with ISP because your router gateway is configured for one of the ISP to sen mail outside your domain.

Smarthost is explained in the above link i posted.
In Exchange environment by default Exchange servers are able to resolve the internal mails and route mails accordingly.

When it is for internet you will need to configure your route rule "*" that means any domain forward it to Smart Host - This can be simple SMTP server which has the capability to transfer mail to next level.

For this it will check with the DNS server for MX record of the external domain. so the DNS server IP which you mention in smart host, will be of your ISP DNS server where it will have MX records of all domains.

So it will through the mail to internet based on the MX record. So in this scenario you will not use DNS forwarder.

In second scenario, if your Exchange bridge head or HUB server will have DNS server configured. And DNS will have been configured as forwarder to resolve internet names. In this case with the help of local DNS itself you will be able to resolve internet name. And your server which is the last server to post mail to internet will able to fetch information locally and send mail to internet mail user. In this case your DNS IP address will be pointing to DNS server which is acting as DNS forwarder.

To understand DNS forwarder go through below link


In Exchange environment by default Exchange servers are able to resolve the internal mails and route mails accordingly.

If i understand you correctly, can you explain how exchange is able to resolve name of the dc as well as other exchange server internally on the network w/o internal dns for domain clients?
Ghousullah:w/o dns, exchange will not able to resolve internal server name, dns contains srv records to locate users in the domain to send them mail, forwarder & root hints in dns are responsible to route the mail to outside domain or through to ISP server to send mail outside its own domain.

DNS is the heart of active directory & w/o dns neither AD nore Exchange can work, either it can be internal mailflow or external mailflow.
Yes. DNS is very much required. I never told DNS is not required. I mentioned here is all about pointing the  DNS server of what kind it is. Whether it will be ISP's DNS server for Edge or it will be internal DNS server with forwarder enabled. That was the one I explained above.
jskfanAuthor Commented:
1- If DNS forwarder is empty, the Root hints will be used. Correct ?

2-SMTP servers: it can be windows 2003 server with SMTP services enabled. Correct??
3-The SMTP servers can be used when we have more than one ISP to route email to. Correct?
4-Smart host is the ISP that has our MX records, and we receive internet email through this smart host and we send through it . correct?
There are two DNS servers used for routing mail to external world.
1. DNS server of ISP where all MX records of different domain will be available.
2. Internal DNS server which needs to be configured as forwarder to forward its request to root hints.

All Servers configured to relay mail are SMTP servers. But the place where it is placed and role it plays makes it difference between Smart host and SMTP server.

The Smart host is also an SMTP server which will use

1) DNS forwarder which is inside the Network or
2) It will use the DNS server of the ISP to find the MX records.

So if your DNS is configured as forwarder you can provide its IP as DNS server for Smarthost. If not you can provide the ISP's DNS server IP as DNS server for it.

To conclude, this the last SMTP server which will sit at the edge of the network to relay mail to outside world.

The above explanation is for your 1st and 4th questions.
 For your second question answer is yes.

For your third question SMTP server can be used as SMART host, but it should able to reach the ISP DNS server. Considering Security, practically the smart host servers are placed in DMZ.
jskfanAuthor Commented:
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