SMTP is a protocol that is used for sending email
messages between servers. Because most email systems that are connected
to the Internet today utilize SMTP as their messaging standard, it is
important to understand how it works with Exchange Server 2007.
Previous
versions of Exchange supported SMTP, but they relied on a service
provided by the underlying Windows operating system. Exchange Server
2007, on the other hand, has its own built-in SMTP server. As a matter
of fact, the installation of Exchange Server 2007 requires that you do
not have the SMTP service already installed on your underlying Windows
platform.
In Exchange, for SMTP traffic to
travel between computers, SMTP connectors are used. SMTP connectors are
logical representations of connections between a source and destination
server. These connectors dictate how Edge Transport servers and Hub
Transport servers communicate with each other, with the Internet, and
with previous versions of Exchange.
There
are two types of SMTP connector in Exchange Server 2007, Send
Connectors and Receive Connectors. Each of these types of connector
represents a one-way connection, and the configuration of the connector
mandates how messages will be transported.
To
secure your Microsoft Exchange Server 2007 environment, you must have
an understanding of these connectors and how to configure them properly.
Connector Topology
For
messages to flow between servers in an Exchange organization, or
between the organization and the Internet, several SMTP connectors must
be in place and properly configured. These connectors are the minimum
that are required for proper end-to-end mail flow. Table 1 lists these connectors.
Table 1. Exchange Server 2007 SMTP Connectors
| Purpose | Type | How Created |
|---|
| Send messages between Hub Transport servers in the organization | Send | Implicit connector that is automatically computed based on the system topology. |
| Send messages from a Hub Transport server to an Edge Transport server | Send | Implicit connector that is automatically computed based on the system topology. |
| Send messages from an Edge Transport server to a Hub Transport server | Send | Implicit connector that is automatically created by the EdgeSync subscription process. |
| Send messages from a Hub Transport server to the Internet | Send | Explicit connector that is created by the administrator and is stored in Active Directory. |
| Send messages from an Edge Transport server to the Internet | Send | Explicit
connector that is either created by the administrator on an Edge
Transport server or automatically created using the EdgeSync
subscription process. |
| Receive messages on a Hub Transport server from another Hub Transport server or from an Edge Transport server | Receive | Explicit
Active Directory connector that is automatically created when the Hub
Transport server role is installed. The connector is stored in Active
Directory as a child object of the server. |
| Receive messages on the Edge Transport server from a Hub Tranport server or from the Internet | Receive | Explicit
connector that is created automatically when the Edge Transport server
role is installed. The connector is stored in ADAM. When the Edge
Transport server is subscribed to an Active Directory site using
EdgeSync, permissions to use this connector are granted to each Hub
Transport server in the site. |
Note
Send and Receive Connectors can be created implicitly, explicitly, or automatically. To say that a connector is created implicitly means that it is computed from the system topology and is not displayed in either the Exchange Management Console or the Exchange Management Shell. A connector that is created explicitly is one that is created when an administrator actively performs a task. Lastly, a connector can be created automatically during the Edge Subscription process.
Understanding Receive Connectors
SMTP
Receive Connectors serve the purpose of acting as incoming connection
points for SMTP traffic and dictates how incoming SMTP communications
are managed on an Exchange 2007 transport server. The Receive Connector
actively listens for incoming connections that match all settings
configured on the connector, such as connections utilizing a particular
port or from a particular IP address range.
Receive Connectors have many configurable limits that can be set, such as the following:
Number of active connections allowed
Maximum incoming message size
Maximum recipients per message
Receive
Connectors are configured on a single server and determine what
particular message traffic that server will listen for. If the Receive
Connector is created on a Hub Transport server, it is stored in Active
Directory as a child object of that server. However, when it is created
on an Edge Transport server, the connector is stored in Active
Directory Application Mode (ADAM).
Understanding Send Connectors
SMTP
Send Connectors are used for relaying outgoing SMTP communications.
Unlike Receive Connectors; Send Connectors are not scoped to a single
server. When an Exchange 2007 server receives an SMTP message that is
addressed to a remote destination, the message is relayed to an
appropriate Send Connector that is configured to handle messages
intended for that destination.
In Active
Directory or in ADAM, a Send Connector is created as an object in a
connectors container. A connector can have more than one source server,
which is defined as a Hub Transport server that is associated with that
connector.
For example, if a Send
Connector is configured to handle message routing to a domain that is
external to the organization, whenever a Hub Transport server receives
a message destined for that remote domain, the message is routed to the
Send Connector to be relayed appropriately.
As
with Receive Connectors, a variety of configuration settings can be
defined by the administrator. Send Connectors can be created and viewed
in either the Exchange Management Console or the Exchange Management
Shell, but the majority of the configuration must be accomplished using
the Exchange Management Shell. Send Connectors are stored in Active
Directory as a configuration object, and can be viewed from the
Exchange Management Console by going to the console tree, selecting
Organization Configuration, and then selecting Hub Transport. Next, in the results pane, select the Send Connectors tab.
