You can use mail flow rules (also known as transport rules) to identify and take action on messages that flow through your Exchange Online organization.
Mail flow rules are similar to the Inbox rules that are available in Outlook and Outlook on the web. The main difference is mail flow rules take action on messages while they're in transit, and not after the message is delivered to the mailbox. Mail flow rules contain a richer set of conditions, exceptions, and actions, which provides you with the flexibility to implement many types of messaging policies.
Mail flow rule components
A mail flow rule is made of conditions, exceptions, actions, and properties:
- Conditions: Identify the messages that you want to apply the actions to. Some conditions examine message header fields (for example, the To, From, or Cc fields). Other conditions examine message properties (for example, the message subject, body, attachments, message size, or message classification). Most conditions require you to specify a comparison operator (for example, equals, doesn't equal, or contains) and a value to match. If there are no conditions or exceptions, the rule is applied to all messages.
For more information about mail flow rule conditions in Exchange Online, see Mail flow rule conditions and exceptions (predicates) in Exchange Online.
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Exceptions: Optionally identify the messages that the actions shouldn't apply to. The same message identifiers that are available in conditions are also available in exceptions. Exceptions override conditions and prevent the rule actions from being applied to a message, even if the message matches all of the configured conditions.
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Actions: Specify what to do to messages that match the conditions in the rule, and don't match any of the exceptions. There are many actions available, such as rejecting, deleting, or redirecting messages, adding additional recipients, adding prefixes in the message subject, or inserting disclaimers in the message body.
How mail flow rules are applied to messages
All messages that flow through your organization are evaluated against the enabled mail flow rules in your organization. Rules are processed in the order listed on the Mail flow > Rules page in EAC, or based on the corresponding Priority parameter value in the PowerShell.
Each rule also offers the option of stopping processing more rules when the rule is matched. This setting is important for messages that match the conditions in multiple mail flow rules (which rule do you want applied to the message? All? Just one?).
Source: https://docs.microsoft.com/en-us/Exchange/security-and-compliance/mail-flow-rules/mail-flow-rules