Microsoft Exchange 2003 – MX Records

Introduction to Microsoft Exchange 2003 – MX Records

If your Exchange 2003 server needs to receive email from the internet, then somewhere, you need to configure MX (mail exchange) records in DNS. When troubleshooting MX records, hold on to the thought that people need to find your email server.

Topics for Exchange 2003 MX Records


MX and DNS for Microsoft Exchange 2003MX Record for Exchange 2003

DNS is a complex topic; so let us start with the basics. Host or A records map the mail server to its IP address. The extra factor that Exchange needs is a DNS record that links the mail server to the email domain name.

Email delivery for your domain

[Host (A)] + [MX Record]
IP — mail server — email domain

Luckily, DNS has an MX record to maintain such a mapping. To check DNS records, launch your server’s DNS snap-in, expand your server icon, Forward Lookup Zones and navigate to your domain folder. Now search for MX records and examine their properties. (See diagram opposite.)

In most instances, you need at least two Exchange servers each with its own MX record. Traditionally MX priorities are set in multiples of 10, and an attempt is made to deliver the email to the server with the lowest priority first.

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Which server will host the MX records?

The next consideration is whose DNS server should host these MX records? The answer depends on how your email domain is registered with InterNic. I stress email domain, because this maybe a different name from that of your Active Directory domain.

Plan A would be for your ISP to have an MX record for your domain pointing to the ISP’s name servers. In this case you would need to periodically download the email from the ISP’s mail server to your Exchange 2003 server.

Plan B would be for you to have a permanent IP address which is continuously connected to the internet. In this case, you could have the MX records on your DNS pointing to your Exchange 2003 servers.

Plan C is a hybrid of the first two plans. Ask your ISP to create MX records for both your server and the ISP’s own servers. In this instance you could assign a lower priority to the MX record for your Exchange server. As a result the email comes to your Exchange servers if the connection is active, or else is delivered temporarily to the ISP, if the internet link is down.

Internal Email

It is possible for email to be delivered internally, within your Exchange Organization, without the need for MX records. The reason is that your own Exchange servers can resolve the delivery request with just an A (Host) DNS record.

For example, suppose you have two Exchange 2003 servers, Cardiff and Swansea. Active Directory will already have DNS A (Host) records for these servers. So when mail is sent by a Cardiff mailbox to guy, Exchange queries Active Directory for guy’s mailbox – answer Swansea, then it queries DNS for the IP address of Swansea. Now it can deliver email to the Swansea Exchange 2003 server. No MX records are required for this scenario.

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DNS for Outgoing EmailDNS MX Records

Naturally, your Outlook clients send all their email to your server. Internal delivery for recipients in your organization is no problem. Your DNS can handle all queries for Exchange servers.

However, any email addressed to external recipient, needs a DNS server with extra information called root hints. Root hints have the IP addresses of internet servers which specialize in top level domains such as .com, .net. For successful delivery, either your DNS server has these root hints configured, or you forward all external email to your ISP’s DNS server. Naturally the ISP’s DNS takes responsibility for resolving the domain names via the root hints.

To configure outgoing DNS, either navigate to the Virtual Server (See diagram), or to the SMTP Routing Connector.

What you are looking for is the Smart host dialog box. This setting would hold the IP address of your ISP’s DNS server. Alternatively, configure the IP address of one of your DNS Servers outside the firewall. When you enter the IP address, strangely, it must be enclosed in square brackets, for example [2xx.221.78.10]

Summary of Exchange MX Records

  1. There are three separate DNS issues for your Exchange 2003 server.
  2. If you are receiving email from the internet, then it is essential to configure MX records for your Exchange 2003 server.
  3. Internal email delivery is accomplished with just A (Host) DNS records.

See Also