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SECURITY

Microsoft Exchange Server 2007 : Server and Transport-Level Security - Exchange Server 2007 SMTP Connectors (part 1) - Connector Topology

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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
PurposeTypeHow Created
Send messages between Hub Transport servers in the organizationSendImplicit connector that is automatically computed based on the system topology.
Send messages from a Hub Transport server to an Edge Transport serverSendImplicit connector that is automatically computed based on the system topology.
Send messages from an Edge Transport server to a Hub Transport serverSendImplicit connector that is automatically created by the EdgeSync subscription process.
Send messages from a Hub Transport server to the InternetSendExplicit connector that is created by the administrator and is stored in Active Directory.
Send messages from an Edge Transport server to the InternetSendExplicit 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 serverReceiveExplicit 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 InternetReceiveExplicit 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.

Other  
  •  Microsoft Exchange Server 2007 : Server and Transport-Level Security - Transport-Level Security Defined
  •  Microsoft Exchange Server 2007 : Exchange Server-Level Security Features (part 2) - Protecting Exchange Server 2007 from Viruses
  •  Microsoft Exchange Server 2007 : Exchange Server-Level Security Features (part 1) - Exchange Server 2007 Antispam Measures
  •  Microsoft Exchange Server 2007 : Components of a Secure Messaging Environment (part 5) - Using Email Disclaimers
  •  Microsoft Exchange Server 2007 : Components of a Secure Messaging Environment (part 4) - Establishing a Corporate Email Policy, Securing Groups
  •  Microsoft Exchange Server 2007 : Components of a Secure Messaging Environment (part 3) - Hardening Windows Server 2003 - Running SCW
  •  Microsoft Exchange Server 2007 : Components of a Secure Messaging Environment (part 2) - Hardening Windows Server 2003 - Using the Microsoft Baseline Security Analyzer
  •  Microsoft Exchange Server 2007 : Components of a Secure Messaging Environment (part 1) - Hardening Windows Server 2003 - Auditing Policies
  •  Microsoft Exchange Server 2007 : Server and Transport-Level Security - Considering the Importance of Security in an Exchange Server 2007 Environment
  •  Security and Windows 8: Keeping Your PC Safe (part 2) - Windows SmartScreen, Using Windows SmartScreen, Action Center Improvements
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