Friday, May 11, 2007

Exchange 2007 Edge Transport server ROLE in SMTP25 NutShell

Here is our corporate mail domain. This SMTP domain is authoritative for and consist of third party hardware appliance mail gateways (iron port, barracuda, etc) being used as smart host mail relay gateways for a corporate level network. This network has over 90.000 mail recipients.

The SMTP mail gateways are configured with same MX record so if there is an SMTP session opens from outside, one of these mail gateways will accept the basic SMTP session and SMTP hand shake will start if the recipient is valid.

After this Smart host will pass the mail back to a Bridge head servers, and bridge head servers will find the location of user home Exchange server and pass the mail to the mailbox server. Mailbox server will locate the user mail store and mailbox and place the mail into it.

In this example we are looking at big picture, we have over 100.000 mail recipients and the SMTP domain spans to corporate exchange serves as well as remote exchange servers.

Mail from:


Host (sender Mail server) from internet does recursive query to locate the authoritative DNS server for

The DNS servers who is authoritative for DNS name space says, here is the IP addresses for mail gateway and hands out four IP address



Authoritative for


Authoritative for


Authoritative for


Authoritative for


The DNS query does not provide road balance. When second query is made to the same authoritative DNS servers, for same SMTP domain the authoritative DNS server will give the answer back to the sender in Roud robin fashion (Roud robin DNS is a method used buy your domain name server to

select a server in a semi-random way.) RFC1123 will try to connect in RONDOM order

The second query may look a table below


Authoritative for


Authoritative for


Authoritative for


Authoritative for


Even though this look like, a Roud robin fashion, it is up to a SENDER mail server who to contact too.

Therefore DNS won't provide any road balancing.

There are three steps to SMTP mail transactions. The transaction is started with a MAIL command which gives the sender identification. A series of one or more RCPT commands follows giving the receiver information. Then a DATA command gives the mail data. And finally, the end of mail data indicator confirms the transaction


SMTP hand-shake starts as soon as sender mail server decides who to send it too.


Telnet 25

Telnet into SMTP port TCP 25

220 Microsoft ESMTP MAIL Service,

Version: 6.0.3790.1830 ready at Fri, 11 May 2007 21:04:40 -0400

220 SMTP service is READY. Connection is ready, Greetings, transmission channel just opened

mail from:

250 2.1.0 OK

250 Sender is Okay

rcpt to:

250 2.1.5

250 Requested mail action okay, completed


354 Start mail input; end with <CRLF>.<CRLF>

Data is going to be inserted, there is no columns, after this Enter Subject with columns, along with subject

Subject: Hello I am Spam King

I came here to Spam you with this e-mail.

Subject is here, hit ENTER

Start writing your body of E-mail here

When you ready to finish

Press on the Keyboard "dot"

Hit Enter

Press one more time "dot"

You are done , go ahead and type

Quit and hit Enter


Now your mail has been accepted by one of the Smart host sitting in DMZ, went through SMTP hand shake, Spam filtering and Virus checking has also done against your mail, your IP address is recorded, and it got checked against RBL list( Spam list), when everything looked good your mail passed to the Exchange Bridge head server. Bridge head server passed the mail to the mailbox server. Mailbox server located the mail store your mailbox is sitting on, and placed in there for you to come back and read it.


When you will read your mail, whenever you open a MAPI client such as outlook, connect back to your mailbox server and pull your mail from there into this application and make it read for you to read, organize or delete.


I have explained how enterprise SMTP domain would operate, with Smart host, bridge head Exchange server and MailBox exchange server. OWA part is out this picture this time

We will migrate,, SMTP domain into Exchange 2007 environment, and there we will replace all third party smart hosts with Edge Transport server. We will cluster them, and use same futures

Even more with Exchange 2007 and I will show you all, how much you could save by doing this.

No more paying for per mailbox, or complicated license fee current hardware third part would charge 5 times for one specific future. (Rip off). Exchange 2007 with 64BIT architecture and its futures is ready to shake the market, in corporate environment, with new OWA, ability to get to your work files (share point integration or file server). Also incredible interface, with UM (unified messaging support) voice mail right into you outlook, is going to be implemented more and more into corporate environments


Part II we will continue



Oz Ozugurlu

1 comment:

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