Anti-spam message headers in Microsoft 365
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In all Microsoft 365 organizations, Exchange Online Protection (EOP) scans all incoming messages for spam, malware, and other threats. The results of these scans are added to the following header fields in messages:
- X-Forefront-Antispam-Report: Contains information about the message and about how it was processed.
- X-Microsoft-Antispam: Contains additional information about bulk mail and phishing.
- Authentication-results: Contains information about SPF, DKIM, and DMARC (email authentication) results.
This article describes what's available in these header fields.
For information about how to view an email message header in various email clients, see View internet message headers in Outlook.
You can copy and paste the contents of a message header into the Message Header Analyzer tool. This tool helps parse headers and put them into a more readable format.
X-Forefront-Antispam-Report message header fields
After you have the message header information, find the X-Forefront-Antispam-Report header. There are multiple field and value pairs in this header separated by semicolons (;). For example:
The individual fields and values are described in the following table.
The X-Forefront-Antispam-Report header contains many different fields and values. Fields that aren't described in the table are used exclusively by the Microsoft anti-spam team for diagnostic purposes.
ARC protocol has the following fields:
|The category of protection policy that's applied to the message:
*Defender for Office 365 only.
An inbound message might be flagged by multiple forms of protection and multiple detection scans. Policies are applied in an order of precedence, and the policy with the highest priority is applied first. For more information, see What policy applies when multiple protection methods and detection scans run on your email.
|The connecting IP address. You can use this IP address in the IP Allow List or the IP Block List. For more information, see Configure connection filtering.
|The source country/region as determined by the connecting IP address, which might not be the same as the originating sending IP address.
|The Directionality of the message:
|The HELO or EHLO string of the connecting email server.
|The message skipped spam filtering because the source IP address was in the IP Allow List. For more information, see Configure connection filtering.
|The IP address wasn't found on any IP reputation list.
|The language that the message was written in as specified by the country code (for example, ru_RU for Russian).
|The PTR record (also known as the reverse DNS lookup) of the source IP address.
|The spam confidence level (SCL) of the message. A higher value indicates the message is more likely to be spam. For more information, see Spam confidence level (SCL).
|The message was identified as phishing and is also marked with one of the following values:
|Filtering was skipped and the message was blocked because it was sent from an address in a user's Blocked Senders list.
For more information about how admins can manage a user's Blocked Senders list, see Configure junk email settings on Exchange Online mailboxes.
|Spam filtering marked the message as nonspam and the message was sent to the intended recipients.
|Filtering was skipped and the message was allowed because it was sent from an address in a user's Safe Senders list.
For more information about how admins can manage a user's Safe Senders list, see Configure junk email settings on Exchange Online mailboxes.
|The message skipped spam filtering and was delivered to the Inbox because the sender was in the allowed senders list or allowed domains list in an anti-spam policy. For more information, see Configure anti-spam policies.
|The message was marked as spam because it matched a sender in the blocked senders list or blocked domains list in an anti-spam policy. For more information, see Configure anti-spam policies.
|The message was marked as nonspam before processing by spam filtering. For example, the message was marked as SCL -1 or Bypass spam filtering by a mail flow rule.
|The message was released from the quarantine and was sent to the intended recipients.
|The message was marked as spam before processing by spam filtering. For example, the message was marked as SCL 5 to 9 by a mail flow rule.
|The message was marked as spam by spam filtering.
|The message was identified as bulk email by spam filtering and the bulk complaint level (BCL) threshold. When the MarkAsSpamBulkMail parameter is
On (it's on by default), a bulk email message is marked as spam (SCL 6). For more information, see Configure anti-spam policies.
|The message matched an Advanced Spam Filter (ASF) setting. To see the X-header value for each ASF setting, see Advanced Spam Filter (ASF) settings.
Note: ASF adds
X-CustomSpam: X-header fields to messages after the messages were processed by Exchange mail flow rules (also known as transport rules), so you can't use mail flow rules to identify and act on messages that were filtered by ASF.
X-Microsoft-Antispam message header fields
The following table describes useful fields in the X-Microsoft-Antispam message header. Other fields in this header are used exclusively by the Microsoft anti-spam team for diagnostic purposes.
|The bulk complaint level (BCL) of the message. A higher BCL indicates a bulk mail message is more likely to generate complaints (and is therefore more likely to be spam). For more information, see Bulk complaint level (BCL) in EOP.
Authentication-results message header
The results of email authentication checks for SPF, DKIM, and DMARC are recorded (stamped) in the Authentication-results message header in inbound messages. The Authentication-results header is defined in RFC 7001.
The following list describes the text that's added to the Authentication-Results header for each type of email authentication check:
SPF uses the following syntax:
spf=<pass (IP address)|fail (IP address)|softfail (reason)|neutral|none|temperror|permerror> smtp.mailfrom=<domain>
spf=pass (sender IP is 192.168.0.1) smtp.mailfrom=contoso.com spf=fail (sender IP is 127.0.0.1) smtp.mailfrom=contoso.com
DKIM uses the following syntax:
dkim=<pass|fail (reason)|none> header.d=<domain>
dkim=pass (signature was verified) header.d=contoso.com dkim=fail (body hash did not verify) header.d=contoso.com
DMARC uses the following syntax:
dmarc=<pass|fail|bestguesspass|none> action=<permerror|temperror|oreject|pct.quarantine|pct.reject> header.from=<domain>
dmarc=pass action=none header.from=contoso.com dmarc=bestguesspass action=none header.from=contoso.com dmarc=fail action=none header.from=contoso.com dmarc=fail action=oreject header.from=contoso.com
Authentication-results message header fields
The following table describes the fields and possible values for each email authentication check.
|Indicates the action taken by the spam filter based on the results of the DMARC check. For example:
|Composite authentication result. Used by Microsoft 365 to combine multiple types of authentication (SPF, DKIM, and DMARC), or any other part of the message to determine whether or not the message is authenticated. Uses the From: domain as the basis of evaluation.
|Describes the results of the DKIM check for the message. Possible values include:
|Describes the results of the DMARC check for the message. Possible values include:
|Domain identified in the DKIM signature if any. This is the domain that's queried for the public key.
|The domain of the
5322.From address in the email message header (also known as the From address or P2 sender). Recipient sees the From address in email clients.
|The reason the composite authentication passed or failed. The value is a three-digit code. For example:
|The domain of the
5321.MailFrom address (also known as the MAIL FROM address, P1 sender, or envelope sender). This email address is used for non-delivery reports (also known as NDRs or bounce messages).
|Describes the results of the SPF check for the message. Possible values include: