Centralized mail transport and sending outgoing messages directly from EXO

Hello,

You can use Rule Based send connectors but in order to implement your migration incrementally you would need to do something similar to this.

1. Create a send connector that will route email to your on-premises exchange and configure the Use of connector to "Only when I have a transport rule setup that redirects message to this connector"

2. Create a transport rule that scopes the messages that you want to have sent to your On-Premises Exchange. For example: you could configure the rule to only use this connector when the messages have an external destination address domain. 

3. Disable Centralized Mail Routing.  You have to do this or the centralized mail routing configuration will continue to send all of your mail to on-premises via your default Hybrid Connector.

Note:

• Once enabled, Exchange Online will send all outbound mail to the on-premises environment first, even if there is a matching send connector in Exchange Online that could handle the mail directly.
• The on-premises Exchange then routes the mail to its final destination, whether that’s an external recipient or another internal mailbox.
  
  
  

So be aware that this is not a "non-change" from a routing perspective.  Mail sent between two of your recipients will no longer be sent to on-premises exchange first and then routed back to you Exchange Online.  It will go directly to the recipient.  You will notice this if you look at your transport logs Exchange on premises or message traces in your Exchange Online Tenant.   

At this point your outbound mail will still be routed to your on-premises service but will do so based on the Mail Flow rule and the new connector you have setup.

Now you can start modifying the mail flow rule to implement your roll out.  For example you could add a " Except If" clause to your transport rule and include specific senders or members of a group in that Except clause. 

Doing so would cause their mail to be sent from Microsoft default sending connector. (this is not visible but its always there). 

This how we implemented this change in our Hybrid Exchange Environment.