This blog post focuses on an issue where your Exchange Online users cannot 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 do not follow the preferred hybrid architecture proposed by the Exchange product group but use a 3rd party software as a centralized email gateway.
Problem
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.
Assumption
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:
- Internet Connector
Use recipients’ domain MX records
Use self-signed certificate
Target address space: *
- Office 365 Connector
Use tenant.mail.protection.outlook.com to route internal email messages
Use self-signed certificate
Target address space: tenant.mail.onmicrosoft.com
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 any information in the Exchange Online message tracking logs.
The following diagram illustrates the setup.
Solution
The solution to this problem is pretty simple. Just use dedicated certificates for each connector targeting Exchange Online.
Change the Internet Connector to a 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:
- Internet Connector
Use recipients’ domain MX records
Use 3rd party certificate
Target address space: *
- Office 365 Connector
Use tenant.mail.protection.outlook.com to route internal email messages
Use self-signed certificate
Target address space: tenant.mail.onmicrosoft.com
The following diagram illustrates the new setup:
Links
- Important notice for Office 365 email customers who have configured connectors
- Mail flow best practices for Exchange Online and Office 365 (overview)
Enjoy!
