We have two tenants TenantA and TenantB. There is a third party filtering service in front of both and routes Internet email to the appropriate tenant after filtering. We want to create a direct connection between the tenants. If I create a connector for TenantA from O365 to Partner Organization's Smart Host TenantB.mail.protection.outlook.com, that seems like it could work.
On the TenantB side, we have a receive connector that is locked down to only receive mail from the third party filtering service and their range of IP's. I could allow EOP's IP ranges, but that seems like it could be abused.
Anyone implement direct connectors between tenants for email exchange involving smart hosts instead of MX records?