How does O365 Mail Flow work for Transport Rules with Multiple Outbound Connectors?

Question

We are implementing a service that will encrypt email for our O365 mail tenant.

• RULE 1: If a specific pattern is found in the email SUBJECT, mail is sent to our mail encryption server and encrypted. Encrypted message is sent BACK to O365 with a Inbound/Receive Connector.
• RULE 2: It sends outgoing mail from specific senders through a particular smarthost.

The desired behavior is that when RULE 1 applies, the mail will be sent to the mail encryption server, which encrypts the mail and sends it BACK to O365 (via its configuration) -- then RULE 2 will kick in and send the encrypted mail outbound per usual.

Problem is that when both rules are turned on the new encryption rule (RULE 1) is being 'hit' in the message trace, but the message is still being sent outbound through the existing smarthost rule (RULE 2).

Its almost like O365 is choosing Rule 1 OR Rule 2 instead of processing Rule 1 AND Rule 2.

Is this the expected behavior of transport rules or am I missing something?

Answer 1

From the relevant microsoft documentation

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?)

I would double check that the rules do not use

• a stopping condition in the first rule
• a particular trigger on the second rule

Also, transport rules can be tested for validation purposes.

Answer 2

Agree with what Mike L'Angelo has said above, each rule has a priority, and the one with the highest priority will be selected first.

Could create one rule for these 2 purpose?