Use mail flow rules to inspect message attachments in Exchange Online

In cloud-based organizations, you can inspect email attachments by setting up Exchange mail flow rules (also known as transport rules). Mail flow rules allow you to examine email attachments as a part of your messaging security and compliance needs. When you inspect attachments, you can then take action on the messages based on the content or characteristics of the attachments. Here are some attachment-related tasks you can do by using mail flow rules:

• Search for files with text that matches a pattern you specify, and add a disclaimer at the end of the message.
• Inspect content within attachments and, if there are any keywords you specify, redirect the message to a moderator for approval before it's delivered.
• Check for messages with attachments that can't be inspected and then block the entire message from being sent.
• Check for attachments that exceed a certain size and then notify the sender of the issue, if you choose to prevent the message from being delivered.
• Check whether the properties of an attached Office document match the values that you specify. With this condition, you can integrate the requirements of your mail flow rules and DLP policies with a non-Microsoft classification system, such as SharePoint or the Windows Server File Classification Infrastructure (FCI).
• Create notifications that alert users if they send a message that matches a mail flow rule.
• Block all messages containing attachments. For examples, see Use mail flow rules for attachment blocking scenarios in Exchange Online.

Exchange Online administrators can create mail flow rules in the Exchange admin center (EAC) at Mail flow > Rules. You need permissions to do this procedure. After you start to create a new rule, you can see the full list of attachment-related conditions by selecting Any attachment under Apply this rule if. The attachment-related options are shown in the following diagram.

For more information about mail flow rules, including the full range of conditions and actions that you can choose, see Mail flow rules (transport rules) in Exchange Online. Customers with Exchange Online Protection (EOP) for cloud protection of on-premises email environments can benefit from the mail flow rules best practices provided in Best practices for configuring Exchange Online Protection for on-premises organizations. If you're ready to start creating rules, see Manage mail flow rules in Exchange Online.

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Inspect the content within attachments

You can use the mail flow rule conditions in the following table to examine the content of message attachments. For these conditions, only the first 2 megabytes (MB) of text extracted from an attachment is inspected. The 2-MB limit refers to the extracted text, not the file size of the attachment. For example, a 4-MB file might contain less than 2 MB of text so that all of the text would be inspected.

To start using these conditions when inspecting messages, you need to add them to a mail flow rule. For more information about creating or changing rules, see Manage mail flow rules in Exchange Online.

Supported file types for mail flow rule content inspection

The following table lists the file types supported by mail flow rules. The system automatically detects file types by inspecting file properties rather than the actual file name extension, thus helping to prevent malicious hackers from being able to bypass mail flow rule filtering by renaming a file extension. A list of file types with executable code that can be checked within the context of mail flow rules is specified later in this article.

Inspect the file properties of attachments

The following conditions can be used in mail flow rules to inspect different properties of files that are attached to messages. To start using these conditions when inspecting messages, you need to add them to a mail flow rule. For more information about creating or changing rules, see Manage mail flow rules.

Supported executable file types for mail flow rule inspection

The mail flow rules use true type detection to inspect file properties rather than merely the file extensions. This approach helps to prevent malicious hackers from being able to bypass your rule by renaming a file extension. The following table lists the executable file types supported by these conditions. If a file that isn't listed here is found, the AttachmentIsUnsupported condition is triggered.

Data loss prevention policies and attachment mail flow rules

To help you manage important business information in emails, you can include any of the attachment-related conditions along with the rules of a data loss prevention (DLP) policy.

DLP policies and attachment-related conditions can help you enforce your business needs by defining those needs as mail flow rule conditions, exceptions, and actions. When you include the sensitive information inspection in a DLP policy, any attachments to messages are scanned for that information only. However, attachment-related conditions such as size or file type aren't included until you add the conditions listed in this article. DLP isn't available with all versions of Exchange; for more information, see Data loss prevention.

For more information

For information on broadly blocking email with attachments, regardless of malware status, see Common attachment blocking scenarios for mail flow rules in Exchange Online.