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Connection filters protect by blocking IP addresses used by well-known spammers. Sender filters add protection by blocking known email addresses and domains that spammers use. The Sender Filter agent fires on the OnMailCommand event when the MAIL FROM data are received for a message. The agent works by comparing the sender email address (extracted from both MAIL FROM and the From: field in the message these are not necessarily the same and are often not in the case of spam) against a list of prohibited addresses that the administrator defines as bad. You can block individual addresses or a complete domain or even any subdomains. Figure 14-9 shows the general approach to creating a table of blocked senders. You can also see that a rule is defined to block messages that are presented without sender information in their header.
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Figure 14-9 Sender Filter agent properties.
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After the agent identifies offending messages, it then has to decide how to handle them. The options are to reject the message immediately with a Sender Denied response or to accept the message with the proviso that it comes from a blocked sender. The Content Filter agent will then take this fact into account when it calculates the SCL value for the message. In all likelihood, the message s SCL value will be increased and might lead to it being blocked at a later point in the anti-spam cycle because its SCL value exceeds the threshold for acceptance.
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The Get-SenderFilterConfig cmdlet reveals the configuration used by the Sender Filter agent. You can compare the results shown here with the properties visible in Figure 14-9.
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Get-SenderFilterConfig
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Name BlockedSenders BlockedDomains BlockedDomainsAndSubdomains Action BlankSenderBlockingEnabled RecipientBlockedSenderAction Enabled ExternalMailEnabled InternalMailEnabled
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SenderFilterConfig {Joe.Spammer@cohowinery.com} {} {cohowinery.com} Reject True Reject True True False
The Set-SenderFilterConfig cmdlet is used to update the configuration. For example, to add a new domain to the prohibited list, we can use a command like this:
Set-SenderFilterConfig -BlockedDomainsAndSubdomains 'cohowinery.com', 'cohovineyard.com','litwareinc.com'
Two points are noteworthy. First, there s an obvious difference between BlockedDomains and BlockedDomainsAndSubdomains because one blocks top-level domains and the other blocks all the subdomains as well. It s easy to miss this subtle but important point when you use tab-complete to cycle through the available parameters for Set-SenderFilterConfig. Second, you have to write out the complete new set of values every time you add or remove an entry for the BlockedDomains, BlockedDomainsAndSubDomains, and BlockedSenders properties. Again, it s easy to miss this and just write the latest domain or sender that you want to block and miss all the values that were present before.
Backscattering
Blocking messages that are presented without a sender is a basic rule of messaging hygiene that has been in place for years. Unfortunately, spammers became wise to this block a long time ago and added sender addresses to messages. This development forced receiving message transfer agents (MTAs) to implement checks with the sender s domain to validate that it is valid (callback verification). If a sender address is invalid, the connection is immediately dropped. To defeat the callback verification check, spammers began to spoof using legitimate addresses that are harvested and sold between spammers. In turn, this led to another problem called backscattering where the spammer exploits two facts. First, they know that they have a legitimate sender address that will be accepted by an MTA, and second, they know that most reputable MTAs will generate a DSN when a message cannot be delivered
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14 Message Hygiene
because it s addressed to an unknown recipient. Spammers can therefore target an email domain with tens of thousands of messages that appear to come from real people with the intention that these messages will never be delivered but instead result in DSNs that are addressed back to the legitimate addresses! The unwary recipient sees the DSN in her inbox and opens it to find out why she has been notified of the undelivered message a message that she never sent and then she reads the content of the undeliverable message. The spammer achieves delivery of content to real users and has circumvented anti-spam protection. The problem for an Exchange organization is that the transport servers have to handle the generation and transmission of the DSNs plus the flood of incoming DSNs that result from spam attacks on other domains. Sender ID protection helps by forcing verification of email servers for domains, but as we discuss shortly, Sender ID is not implemented universally. The solution is often found through Bounce Address Tag Verification (BATV), a method to suppress backscattering by stamping outbound messages with verifiable tags to attest that these messages come from real users. BATV is generally available across a range of popular MTAs including SendMail, Exim, and Postfix. The Exchange 2010 anti-spam agents do not include BATV, but add-on products such as Microsoft Forefront Protection for Exchange 2010 (FPE) include optional backscatter filters that can be enabled. In the case of FPE, the backscatter filter is implemented as an outbound agent to stamp messages as they pass through the transport categorizer and an inbound agent to examine messages as they are processed through the SMTP pipeline. Implementing protection against backscattering is not absolutely essential, but the development of filters to suppress the backscattered messages is a good example of the kind of feature that exists in add-on products that isn t available in the Exchange 2010 anti-spam agents that you should consider when you make decisions about how to protect your servers.
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