Using DNS to Route SMTP Mail in Exchange Server 2003

Simple Mail Transfer Protocol (SMTP) has become the standard Internet protocol for electronic mail. Commonly used on Unix and Linux environments, and more recently in Windows, SMTP is used not only for mail delivery across the Internet, but also used within Active Directory as an alternative transport mechanism for site traffic.

Domains that want to participate in electronic mail exchange need to set up MX record(s) for their published zone. This advertises the system that will handle mail for the particular domain, so that SMTP mail will find the way to its destination.

Using DNS in Exchange 2003

Microsoft Exchange Server 2003 uses DNS exclusively for name resolution. In addition to talking to a DNS server for local name lookup, it also uses DNS to communicate over the Internet via SMTP mail services.

Each user has to authenticate to the Active Directory in order to access an Exchange mailbox. Exchange Server 2003 itself has information about authenticating other servers in the domain. This information can be found in Exchange System Manager under the Server Properties, Directory Access tab. The Exchange server obtains this information from the DNS server.

Understanding SMTP Mail Routing

Email is probably the most widely used TCP/IP and Internet application today, with the possible exception of the World Wide Web. SMTP defines a set of rules for addressing, sending and receiving mail between systems, based on the model of communication shown in Figure 7.4. As a result of a user mail request, the SMTP sender establishes a two-way connection with the SMTP receiver. The SMTP receiver can be either the ultimate destination or an intermediate (mail gateway). The SMTP sender generates commands that are replied to by the receiver. All this communication takes place over TCP port 25. When the connection is established, a series of commands and replies are exchanged between the client and server. This connection is similar to a phone conversation, and the commands and responses are equivalent to verbal communication.

Figure 7.4. SMTP communications.

NOTE

In various implementations , there is a possibility of exchanging mail between the TCP/IP SMTP mailing system and the locally used mailing systems. These applications are called mail gateways or mail bridges . Sending mail through a mail gateway may alter the end-to-end delivery specification, because SMTP guarantees delivery only to the mail gateway host, not to the real destination host, which is located beyond the TCP/IP network. When a mail gateway is used, the SMTP end-to-end transmission is host-to-gateway, gateway-to-host, or gateway-to-gateway; the behavior beyond the gateway is not defined by SMTP.

Examining Client DNS Use for Exchange

Before users can access their mailboxes on an Exchange server, they must be authenticated. Authentication requires a DNS lookup in order to locate a domain controller on which the users' accounts can be authenticated.

Clients normally cannot deliver messages directly to destination mail hosts . They typically use a mail server to relay messages to destinations. Using SMTP, clients connect to a mail server, which first verifies that the client is allowed to relay through this server, and then accepts the message destined for other domains.

A client uses DNS to resolve the name of a mail server. For example, when configuring an Outlook mail client to connect to an Exchange server, only the short name and not the FQDN is used to connect to the server. The short name is resolved by DNS to the FQDN of the Exchange server to which the client is connected.