de-DEen-GB
 
rss

Just can't get enough of IT

This blog is about mostly anything in IT. But the primary focuses are Microsoft Technologies like Exchange, Office 365, Azure and Cloud Security.

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.

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 receipients 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 an any information in the Exchange Online message tracking logs.

The following diagram illustrates the setup.

Broken mail flow to other Exchange Online tenants

Solution

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:

  • Internet Connector
    Use receipients 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:

Workign mail flow to other Exchange Online tenants

 

Links

Enjoy!

 

 

Read More »
On September 5, 2016
0 Comment
1568 Views

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

  • Front End Transport service
  • Transport service
  • Mailbox Transport service
  • Mailbox Transport Delivery service

Exchange Server 2016 Mail Flow with Ports

By default Exchange Server 2016 implements the following receive connectors

  • Front End Transport service
    • Default Frontend SERVER, TCP 25
    • Outbound Proxy Frontend SERVER, TCP 717
    • Client Frontend SERVER, TCP 587
  • Transport service
    • Default SERVER, TCP 2525
      Server SMTP connections connected to TCP 25 are proxied to this connector
    • Client Proxy SERVER, TCP 465
      Client submission connections connected to TCP 587 are proxied to this connector
  • Mailbox Transport service
    • SERVER\Default Mailbox Delivery SERVER, TCP 475 (hidden)

Cross server SMTP communication occurs on either TCP 2525 or TCP 475.

Enjoy Exchange Server!

Link

 

 

Read More »