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Understanding Sender Filtering

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Applies to: Exchange Server 2010

The Sender Filter agent is an anti-spam filter that's enabled on computers that have the Microsoft Exchange Server 2010 Edge Transport server role installed. The Sender Filter agent relies on the MAIL FROM: SMTP header to determine what action, if any, to take on an inbound e-mail message.

When you configure anti-spam filters on an Edge Transport server, the filters act on messages cumulatively to reduce the number of unsolicited messages that enter the enterprise. For more information about how to plan and deploy the anti-spam features, see Understanding Anti-Spam and Antivirus Functionality.

The Sender Filter agent acts on messages from specific senders outside the organization. Administrators of Edge Transport servers maintain a list of senders who are blocked from sending messages to the organization. As an administrator, you can block single senders (kim@contoso.com), whole domains (*.contoso.com), or domains and all subdomains (*.contoso.com). You can also configure what action the Sender Filter agent should take when a message that has a blocked sender is found. You can configure the following actions:

  • The Sender Filter agent rejects the SMTP request with a "554 5.1.0 Sender Denied" SMTP session error and closes the connection.
  • The Sender Filter agent accepts the message and updates the message to indicate that the message came from a blocked sender. Because the message came from a blocked sender and it's marked as such, the Content Filter agent will use this information when it calculates the spam confidence level (SCL).

You can use the Exchange Management Console (EMC) or the Exchange Management Shell to designate blocked senders and to define how the Sender Filter agent should act on messages from blocked senders. For more information about how to configure the Sender Filter agent, see Configure Sender Filtering Properties.

Important

The MAIL FROM: SMTP headers can be spoofed. Therefore, you shouldn't rely on the Sender Filter agent only. Use the Sender Filter agent and the Sender ID agent together. The Sender ID agent uses the originating IP address of the sending server to try to verify that the domain in the MAIL FROM: SMTP header matches the domain that's registered. For more information about the Sender ID agent, see Understanding Sender ID.

Looking for management tasks related to managing transport servers? See Managing Transport Servers.

Using the Sender Filter Agent to Block Messages

By default, sender filtering is enabled on the computer that has the Edge Transport server role installed for inbound messages that come from the Internet but aren't authenticated. These messages are handled as external messages. You can disable the Sender Filter agent in individual computer configurations by using the EMC or the Shell. For more information, see Enable or Disable Sender Filtering.

When you enable the Sender Filter agent on a computer, the Sender Filter agent filters all messages that come through all Receive connectors on that computer. As noted earlier in this topic, only messages that come from external sources are filtered. External sources are defined as non-authenticated sources. These are considered anonymous Internet sources.

For more information about how to configure Receive connectors and how message source categories are determined, see Understanding Receive Connectors.

As a best practice, you shouldn't filter e-mail messages from trusted partners or from inside your organization. When you run anti-spam filters, there's always a chance that the filters will detect false positives. You should enable anti-spam agents to run only on messages from potentially untrusted and unknown sources. This will reduce the chance that anti-spam filters will mishandle legitimate messages. You can enable and disable the Sender Filter agent to run on messages from any source by using the Shell. For more information, see Set-SenderFilterConfig.

You can configure the Sender Filter agent to block inbound messages that don't specify a sender and domain in the MAIL FROM: SMTP header. You can use this feature to prevent non-delivery report (NDR) attacks on the Exchange server. Most legitimate SMTP messages come from SMTP servers that provide a sender and domain in the MAIL FROM: SMTP command.

Specifying the Block Action

After you've specified blocked senders and domains, you must specify how you want the Sender Filter agent to act on messages from blocked senders and domains. We recommend that you reject the messages. When you use the Sender Filter agent on which all blocked e-mail addresses and domains are specified by the Edge Transport server administrator, the chance of false positives is relatively less than when you use other anti-spam agents. For example, the Content Filter agent is an anti-spam agent that relies on many different variables to determine whether a message is spam.

There are only two scenarios in which legitimate messages may be rejected by the Sender Filter agent:

  • If you mistype an e-mail address or domain name, the wrong sender may be blocked.
  • If a domain name is reregistered to a legitimate company after you add the domain to your Blocked Senders list, you will unintentionally block legitimate messages.

In either of these cases, it may still make sense to reject the messages.