This blog post focusses on an issue where your Exchange Online users are not able to send emails to other Exchange Online recipients outside of your organization when using a 3rd Party Centralized Email Flow Setup. The term 3rd Party Centralized Email Flow Setup describes a solution where you not follow the preferred hybrid architecture proposed by the Exchange product group, but use a 3rd party software as a centralized email gateway.
You have followed the recommendation to secure the Exchange Online inbound connector for your on-premises email servers by using a certificate name or the remote IP address of your on-premises email gateway.
The on-premises email security gateway utilizes a self-signed certificate to secure TLS connections. The gateway is configured to use two different send connector setups:
In this case Exchange Online Protection (EOP) will not be able to differentiate between tenant internal inbound mail flow and mail flow targeted to other tenants. Therefore, email messages sent from your Exchange Online users to recipients located in other Exchange Online tenants will be discarded.
Interestingly enough, this will happen silently. Your gateway solution will log a successful delivery to Exchange Online Protection. The tenant administrator of the recipient domain will not find an any information in the Exchange Online message tracking logs.
The following diagram illustrates the setup.
The solution for this problem is pretty simple. Just use dedicated certificates for each connector targetting Exchange Online.
Change the Internet Connector to fully trusted 3rd party certificate. In this case you are not required to modify the existing Exchange Online inbound connector setup.
The new connector configurations are:
The following diagram illustrates the new setup:
Microsoft TechNet provides detailed documentation on Exchange Server 2016 mail flow and the transport pipeline. That article helps you to
The detailed diagram showing the Exchange Server 2016 transport pipeline in the TechNet documentation does not show the TCP ports being used by the Exchange Server 2016 components.
The following diagram is an updated version of the original diagram showing the TCP ports being used by
By default Exchange Server 2016 implements the following receive connectors
Cross server SMTP communication occurs on either TCP 2525 or TCP 475.
Enjoy Exchange Server!