Mail flow in Hybrid w/Appliance

Hi Daniel Thompson,

For incoming, you would need to look at white-listing the IPs of the appliance in Exchange Online so it doesn't treat them as spam., then point the appliance away from the current Exchange 2010 and onto the MX record of your tenant. I'd also look to set up a connector to reject any email that doesn't come from your appliance. 

For outbound, you'll want to set up a connector to route through all of the email to your appliance - something like from Office 365 to partner organisation. I'm not familiar with your particular appliance, but you might have to do some config on that so it expects emails to come from Office365 rather than your old Exchange box.

https://www.azure365pro.com/configuring-mimecast-with-office-365/ - this may help. It's for Mimecast but once you get past all the directory sync stuff at the beginning it should be the same principle for mail routing with your own device.

Hope this helps,

Mark