What is Microsoft's SmartScreen spam filter?

SmartScreen® is filtering technology Microsoft developed and applies across its email platforms to provide customers with the latest anti-spam and anti-phishing tools and innovations throughout its network. These products include, Exchange, Office 365, and others.

As of November 1, 2016, Microsoft stopped generating updates for SmartScreen spam filters in Exchange Server 2016 and earlier (2013, 2010, 2007), Outlook 2016 for Windows and earlier (2013, 2010, 2007) and Outlook 2011 for Mac. It is still used for basic protection, but its effectiveness will degrade over time. It was replaced by Exchange Online Protection (EOP), but Microsoft will merge some of the SmartScreen technology with EOP to leverage its spam fighting capabilities.

Microsoft SmartScreen uses SmartScreen technology to screen email so it can identify and separate junk email from legitimate email. Based on Microsoft's machine-learning technology, the SmartScreen content filter learns from:

  • Known spam and phishing threats
  • User feedback
  • users who have opted into the junk classification program

This data is also an input to sender reputation.

Machine learning refers to the probability-based algorithms that examine the characteristics of email and are used to distinguish legitimate email from junk. Ongoing feedback from customers in the junk classification program helps ensure that the SmartScreen technology is continually trained and improved.

How it works
When an external user sends email to an account, SmartScreen evaluates the content of the message and assigns the message a rating based on the probability that the message is junk. This rating is stored as a message property called a spam confidence level (SCL) within the message itself. The SCL rating stays with the message as it is sent to other anti-spam tools within

Rules inside are set to handle email messages with various SCL ratings:

  • Messages with an SCL rating below a certain threshold are considered spam and deleted instead of sent to a user's junk folder.
  • Messages with an SCL rating higher than the threshold are delivered to the user's junk folder instead of the inbox.


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