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 doesn't 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's passed in the HELO statement. The typical behavior of a legitimate user may be to use a different, but 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 doesn't 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's returned by DNS is the domain name that's registered by using the domain naming authority for that IP address. Sender reputation compares the domain name that's returned by DNS to the domain name that the sender submitted in the HELO/EHLO SMTP command. If the domain names don't 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 from 0 through 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 overall SRL.
  
  
• 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 the proxies.
  
  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.
    
    
  Frequently with insufficient logging, open proxies provide an ideal way for malicious users to hide their true identities and launch denial of service attacks (DoS) or send spam. As more proxy servers are configured to be open by default, open proxies have become more common. Additionally, malicious users can use multiple open proxies together to hide the sender's originating IP address.
  
  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.
  
  

Sender reputation weighs each of these statistics and calculates an SRL for each sender. The SRL is a number from 0 through 9 that predicts the probability that a specific sender is a spammer or otherwise malicious user. A value of 0 indicates that the sender isn't 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 from 0 through 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:

• Reject
  
  
• Delete and archive
  
  
• Accept and mark as a blocked sender
  
  

If a sender is included in the IP Block list or Microsoft 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 the Microsoft Exchange Anti-spam Update Service.

By default, the Edge Transport server sets a rating of 0 for senders that haven't been analyzed. After a sender has sent 20 or more messages, sender reputation calculates an SRL that's based on the statistics listed earlier in this topic.

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