Applies to: Exchange Server 2007 SP3, Exchange Server
2007 SP2, Exchange Server 2007 SP1, Exchange Server 2007
Topic Last Modified: 2006-09-18
Sender Reputation is anti-spam functionality that is enabled on computers that have the Microsoft Exchange Server 2007 Edge Transport server role installed to block messages according to many characteristics of the sender. Sender reputation relies on persisted data about the sender to determine what action, if any, to take on an inbound message.
When you configure anti-spam agents on an Edge Transport server, the agents act on messages cumulatively to reduce the number of unsolicited messages that enter the organization. For more information about how to plan and deploy the anti-spam agents, see Anti-Spam and Antivirus Functionality.
Calculation of the Sender Reputation Level
A sender reputation level (SRL) is calculated from the following statistics:
- HELO/EHLO analysis The HELO and EHLO
SMTP commands are intended to provide the domain name, such as
Contoso.com, or IP address of the sending SMTP server to the
receiving SMTP server. Malicious users, or spammers,
frequently forge the HELO/EHLO statement in various ways. For
example, they type an IP address that does not match the IP address
from which the connection originated. Spammers also put domains
that are known to be locally supported at the receiving server in
the HELO statement in an attempt to appear as if the domains are in
the organization. In other cases, spammers change the domain that
is passed in the HELO statement. The typical behavior of a
legitimate user may be to use a different, but a relatively
constant, set of domains in their HELO statements.
Therefore, analysis of the HELO/EHLO statement on a per-sender basis may indicate that the sender is likely to be a spammer. For example, a sender that provides many different unique HELO/EHLO statements in a specific time period is more likely to be a spammer. Senders who consistently provide an IP address in the HELO statement that does not match the originating IP address as determined by the Connection Filter agent are also more likely to be spammers, as are remote senders who consistently provide a local domain name, which is in the same organization as the Edge Transport server, in the HELO statement.
- Reverse DNS lookup Sender reputation
also verifies that the originating IP address from which the sender
transmitted the message matches the registered domain name that the
sender submits in the HELO or EHLO SMTP command.
Sender reputation performs a reverse DNS query by submitting the originating IP address to DNS. The result that is returned by DNS is the domain name that is registered by using the domain naming authority for that IP address. Sender reputation compares the domain name that is returned by DNS to the domain name that the sender submitted in the HELO/EHLO SMTP command. If the domain names do not match, the sender is likely to be a spammer, and the overall SRL rating for the sender is adjusted upward.
The Sender ID agent performs a similar task, but the success of the Sender ID agent relies on legitimate senders to update their DNS infrastructure to identify all the e-mail-sending SMTP servers in their organization. By performing a reverse DNS lookup, you can help identify potential spammers.
- Analysis of SCL ratings on messages from a particular
sender When the Content Filter agent processes
a message, it assigns a spam confidence level (SCL) rating to the
message. The SCL rating is a number between 0 and 9. A higher SCL
rating indicates that a message is more likely to be spam. Data
about each sender and the SCL ratings that their messages yield is
persisted for analysis by sender reputation. Sender reputation
calculates statistics about a sender according to the ratio between
all messages from that sender that had a low SCL rating in the past
and all messages from that sender that had a high SCL rating in the
past. Additionally, the number of messages that have a high SCL
rating that the sender has sent in the last day is applied to the
- Sender open proxy test An open
proxy is a proxy server that accepts connection requests from
anyone anywhere and forwards the traffic as if it originated from
the local hosts. Proxy servers relay TCP traffic through firewall
hosts to provide user applications transparent access across the
firewall. Because proxy protocols are lightweight and independent
of user application protocols, proxies can be used by many
different services. Proxies can also be used to share a single
Internet connection by multiple hosts. Proxies are usually set up
so that only trusted hosts inside the firewall can cross through
Open proxies can exist because of either of the following conditions:
- Unintentional misconfiguration
- Malicious Trojan horse programs. A Trojan horse program
is a program that masquerades as another common program in an
attempt to receive information.
When sender reputation performs an open proxy test, it does so by formatting an SMTP request in an attempt to connect back to the Edge Transport server from the open proxy. If an SMTP request is received from the proxy, sender reputation verifies that the proxy is an open proxy and updates the open proxy test statistic for that sender.
- Unintentional misconfiguration
Sender reputation weighs each of these statistics and calculates an SRL for each sender. The SRL is a number between 0 and 9 that predicts the probability that a specific sender is a spammer or otherwise malicious user. A value of 0 indicates that the sender is not likely to be a spammer; a value of 9 indicates that the sender is likely to be a spammer.
You can configure a block threshold between 0 and 9 at which sender reputation issues a request to the Sender Filter agent, and, therefore, blocks the sender from sending a message into the organization. When a sender is blocked, the sender is added to the Blocked Senders list for a configurable period. How blocked messages are handled depends on the configuration of the Sender Filter agent. The following actions are the options for handling blocked messages:
- Delete and archive
- Accept and mark as a blocked sender
If a sender is included in the Microsoft Block List or IP Reputation Service, sender reputation issues an immediate request to the Sender Filter agent to block the sender. To take advantage of this functionality, you must enable and configure Microsoft Exchange Anti-spam Update Service.
By default, the Edge Transport server sets a rating of 0 for senders that have not been analyzed. After a sender has sent 20 or more messages, sender reputation calculates an SRL that is based on the statistics listed earlier in this topic.
Use of the SRL
Sender Reputation acts on messages during two phases of the SMTP session:
- At the MAIL FROM: SMTP command Sender
reputation acts on a message only if the message was blocked or
otherwise acted on by the Connection Filter agent, Sender Filter
agent, Recipient Filter agent, or Sender ID agent. In this case,
sender reputation retrieves the sender's current SRL rating from
the sender profile that is persisted about that sender in the Edge
Transport database. After this rating is retrieved and evaluated,
the Edge Transport server configuration dictates the behavior that
occurs at a particular connection according to the block
- After the "end of data" SMTP
command The end of data transfer (_EOD)
SMTP command is given when all the actual message data is sent. At
this point in the SMTP session, many of the anti-spam agents have
processed the message. As a by-product of anti-spam processing, the
statistics that sender reputation relies on are updated. Therefore,
sender reputation has the data to calculate or recalculate an SRL
rating for the sender.
For more information, see Configuring Sender Reputation.
For More Information
For more information about anti-spam features in Outlook 2007, see the following topics:
For more information about how to configure sender reputation, see the following topics: