Anti-spam protection in EOP


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This topic is intended for admins. For end-user topics, see Overview of the Junk Email Filter and Learn about junk email and phishing.

In Microsoft 365 organizations with mailboxes in Exchange Online or standalone Exchange Online Protection (EOP) organizations without Exchange Online mailboxes, email messages are automatically protected against spam (junk email) by EOP.

Microsoft's email safety roadmap involves an unmatched cross-product approach. Anti-spam and anti-phishing technology in EOP is applied across our email platforms to provide users with the latest anti-spam and anti-phishing tools and innovations throughout the network. The goal for EOP is to offer a comprehensive and usable email service that helps detect and protect users from junk email, fraudulent email threats (phishing), and malware.

As email use has grown, so has email abuse. Unmonitored junk email can clog inboxes and networks, affect user satisfaction, and hamper the effectiveness of legitimate email communications. That's why Microsoft continues to invest in anti-spam technologies. Simply put, it starts by containing and filtering junk email.


The following anti-spam technologies are useful when you want to allow or block messages based on the message envelope (for example, the sender's domain or the source IP address of the message). To allow or block messages based on payload (for example, URLs in the message or attached files), then you should use the Tenant Allow/Block List portal.

Anti-spam technologies in EOP

To help reduce junk email, EOP includes junk email protection that uses proprietary spam filtering technologies to identify and separate junk email from legitimate email. EOP spam filtering learns from known spam and phishing threats and user feedback from our consumer platform, Ongoing feedback from EOP users in the junk email classification program helps ensure that the EOP technologies are continually trained and improved.

The anti-spam settings in EOP are made of the following technologies:

  • Connection filtering: Identifies good and bad email source servers early in the inbound email connection via the IP Allow List, IP Block List, and the safe list (a dynamic but non-editable list of trusted senders maintained by Microsoft). You configure these settings in the connection filter policy. Learn more at Configure connection filtering.

  • Spam filtering (content filtering): EOP uses the following spam filtering verdicts to classify messages:

    • Spam
    • High confidence spam
    • Phishing
    • High confidence phishing
    • Bulk, because the message source met or exceeded the bulk complaint level (BCL) threshold.

    You can configure the actions to take based on these verdicts. Quarantine policies determine what users are allowed to do to quarantined messages, and whether they receive notifications for their quarantined messages. For more information, see Configure anti-spam policies in Microsoft 365.


    By default, messages that are marked as Spam are delivered and moved to the recipient's Junk Email folder. However, in hybrid environments where EOP protects on-premises Exchange mailboxes, you need to configure two mail flow rules (also known as transport rules) in your on-premises Exchange organization to recognize the EOP spam headers that are added to messages. For details, see Configure EOP to deliver spam to the Junk Email folder in hybrid environments.

  • Outbound spam filtering: EOP also checks to make sure that your users don't send spam, either in outbound message content or by exceeding outbound message limits. For more information, see Configure outbound spam filtering in Microsoft 365.

  • Spoof intelligence: For more information, see Anti-spoofing protection in EOP.

Manage errors in spam filtering

It's possible that good messages can be identified as spam (also known as false positives), or that spam can be delivered to the Inbox (also known as false negatives). You can use the suggestions in the following sections to find out what happened and help prevent it from happening in the future.

Here are some best practices that apply to either scenario:

  • Always report misclassified messages to Microsoft. For more information, see Report messages and files to Microsoft.

  • Examine the anti-spam message headers: These values tell you why a message was marked as spam, or why it skipped spam filtering. For more information, see Anti-spam message headers.

  • Point your MX record to Microsoft 365: In order for EOP to provide the best protection, we always recommend that you have email delivered to Microsoft 365 first. For instructions, see Create DNS records at any DNS hosting provider for Microsoft 365.

    If the MX record points to some other location (for example, a third-party anti-spam solution or appliance), it's difficult for EOP to provide accurate spam filtering. In this scenario, you need to configure Enhanced Filtering for connectors (also known as skip listing). For instructions, see Enhanced Filtering for Connectors in Exchange Online.

  • Use email authentication: If you own an email domain, you can use DNS to help ensure that messages from senders in that domain are legitimate. To help prevent spam and unwanted spoofing in EOP, use all of the following email authentication methods:

  • Verify your bulk email settings: The bulk complaint level (BCL) threshold that you configure in anti-spam policies determines whether bulk email (also known as gray mail) is marked as spam. The PowerShell-only setting MarkAsSpamBulkMail that's on by default also contributes to the results. For more information, see Configure anti-spam policies in Microsoft 365.

Prevent the delivery of spam to the Inbox

For steps to help prevent false negatives, see How to handle malicious emails that are delivered to recipients (False Negatives), using Microsoft Defender for Office 365.

You can also use the following suggestions to help prevent false negatives:

  • Verify your organization settings: Watch out for settings that allow messages to skip spam filtering (for example, if you add your own domain to the allowed domains list in anti-spam policies). For our recommended settings, see Recommended settings for EOP and Microsoft Defender for Office 365 security and Create safe sender lists.

  • Use the available blocked sender lists: For information, see Create blocked sender lists.

  • Unsubscribe from bulk email If the message was something that the user signed up for (newsletters, product announcements, etc.) and contains an unsubscribe link from a reputable source, consider asking them to unsubscribe.

  • Standalone EOP: create mail flow rules in on-premises Exchange for EOP spam filtering verdicts: In hybrid environments where EOP protects on-premises Exchange mailboxes, you need to configure mail flow rules (also known as transport rules) in on-premises Exchange. These mail flow rules translate the EOP spam filtering verdict so the junk email rule in the mailbox can move the message to the Junk Email folder. For details, see Configure EOP to deliver spam to the Junk Email folder in hybrid environments.

Prevent good email from being identified as spam

For steps to help prevent false positives, see How to handle Legitimate emails getting blocked (False Positive), using Microsoft Defender for Office 365.

You can also use the following suggestions to help prevent false positives:

  • Verify the user's Outlook Junk Email Filter settings:

    • Verify the Outlook Junk Email Filter is disabled: When the Outlook Junk Email Filter is set to the default value No automatic filtering, Outlook doesn't attempt to classify messages as spam. When it's set to Low or High, the Outlook Junk Email Filter uses its own SmartScreen filter technology to identify and move spam to the Junk Email folder, so you could get false positives. Microsoft stopped producing spam definition updates for the SmartScreen filters in Exchange and Outlook in November 2016. The existing SmartScreen spam definitions were left in place, but their effectiveness is likely degraded.
    • Verify the Outlook 'Safe Lists Only' setting is disabled: When this setting is enabled, only messages from senders in the user's Safe Senders list or Safe Recipients list are delivered to the Inbox. Email from everyone else is automatically moved to the Junk Email folder.

    For more information about these settings, see Configure junk email settings on Exchange Online mailboxes in Microsoft 365.

  • Use the available safe sender lists: For information, see Create safe sender lists.

  • Verify users are within the sending and receiving limits as described in Receiving and sending limits in the Exchange Online service description.

  • Standalone EOP: use directory synchronization: If you use standalone EOP to help protect your on-premises Exchange organization, you should sync user settings with the service by using directory synchronization. This configuration ensures that your users' Safe Senders lists are respected by EOP. For more information, see Use directory synchronization to manage mail users.

Anti-spam legislation

At Microsoft, we believe that the development of new technologies and self-regulation requires the support of effective government policy and legal frameworks. The worldwide spam proliferation has spurred numerous legislative bodies to regulate commercial email. Many countries/regions now have spam-fighting laws in place. The United States has both federal and state laws governing spam, and this complementary approach is helping to curtail spam while enabling legitimate e-commerce to prosper. The CAN-SPAM Act expands the tools available for curbing fraudulent and deceptive email messages.