Need an Exchange Transport Rule to identify mail sent to an Alias or Plus address

You cannot use mail flow rules for that. Outlook's Inbox rules do offer some more flexibility, and can potentially be used for this scenario. Though the type of rule might need to be client-side, i.e. it will only run when Outlook is opened.

Alternatively, you can use Flow to the same effect.

Hi @Jon Martin

Good day, and thank you for reaching out. 

Based on my research, Exchange does not expose the “original recipient” (alias or plus address) as a direct or usable variable within transport (mail flow) rules. This information is only available indirectly through message headers. 

I understand the logic behind your approach, however, the objects you are looking for (such as RecipientAddressType: Original and Resolved) are not available as conditions or actions in Exchange mail flow rules. 

To better understand your requirement, may I know whether you are trying to identify the specific alias or plus address used for each individual user, or whether your goal is to identify emails sent to aliases at a group or department level? 

In the meantime, I have found some relevant articles that might help, you could refer vIa: 

Exchange Online Mail Flow Rules for Aliases | mjcb.ca 

How to identify an email sent to an alias using rules - Collset Ltd - powered by HappyFox 

Note: This information is provided as a convenience to you. These sites are not controlled by Microsoft, and Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please ensure that you fully understand the risks before using any suggestions from the above link. 

Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution. 

Note: Please follow the steps in [our documentation] to enable e-mail notifications if you want to receive the related email notification for this thread. 

Mail flow (transport) rules can examine and act on the message as it moves through the transport pipeline, but the available predicates and actions do not expose separate “original vs. resolved” recipient address types in the way described.

From the available documentation:

• Mail flow rules in Exchange Online and Exchange Server can inspect sender, recipient, headers, subject, body, and attachments using built‑in conditions and exceptions, including pattern matching with regular expressions.
• Actions can add disclaimers, headers, or modify the message, but they operate on data that the rule engine can already see (for example, the current recipient address or header values), not on an internal distinction between an “original” plus/alias address and a “resolved” primary address.
• Regular expressions can be used in predicates to match text patterns in addresses or headers, but only against the values that are actually present in those fields at rule‑evaluation time.

The documentation does not describe any predicate or token that exposes both the original envelope/alias/plus address and the resolved primary SMTP address in a way that would allow a rule to compare them and then insert the original address into a disclaimer. It also does not document a built‑in variable that could be used in a disclaimer action to output such an “original recipient” value.

Therefore, based on the documented capabilities, a transport rule that:

• Detects that the message was sent to a plus/alias address but delivered to a primary address, and
• Inserts the original plus/alias address dynamically into a disclaimer

is not supported with the built‑in mail flow rule predicates and actions.

Custom handling of plus/alias addresses would require an approach outside what is documented for mail flow rules (for example, application logic or a separate processing component), not a standard Exchange Online transport rule.

References:

• Mail flow rules (transport rules) in Exchange Online
• Mail flow rules in Exchange Server
• Mail flow rule actions in Exchange Online
• Regular expressions in transport rules
• Mail flow rule components