The primary protocol for sending email on the Internet today is known as Simple Mail Transfer Protocol, or SMTP. SMTP has been used for quite some time in UNIX and Linux environments, and has been incorporated into 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.

Understanding SMTP Mail Routing

Email is arguably the most widely used TCP/IP and Internet application today. SMTP defines a set of rules for addressing, sending, and receiving mail between systems. 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.

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 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.