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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!

 

 

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Exchange Server 2013 Exchange Server 2016Problem

When you integrate Skype for Business Server instant messaging with Exchange Server 2013 or Exchange Server 2016 you might encounter the following error in the OWA InstantMessaging log files.

ERROR:UCWEB Failure: Code=TlsFailure, SubCode=TlsRemoteDisconnected, Reason=\r\n
Microsoft.Rtc.Internal.UCWeb.Utilities.UCWException: Unknown error (0x80131500) 
---> Microsoft.Rtc.Signaling.TlsFailureException: Unknown error (0x80131500) 
---> Microsoft.Rtc.Internal.Sip.RemoteDisconnectedException: Peer disconnected while outbound capabilities negotiation was in progress 
---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host\r\n   
at System.Net.Sockets.Socket.EndReceive(IAsyncResult asyncResult)\r\n   
at Microsoft.Rtc.Internal.Sip.TcpTransport.OnReceived(Object arg)\r\n   
--- End of inner exception stack trace ---\r\n   
--- End of inner exception stack trace ---\r\n   
at Microsoft.Rtc.Signaling.SipAsyncResult`1.ThrowIfFailed()\r\n   
at Microsoft.Rtc.Signaling.Helper.EndAsyncOperation[T](Object owner, IAsyncResult result)\r\n   
at Microsoft.Rtc.Internal.UCWeb.UCWAuthenticatedEndpoint.OotyUserEndpointEstablish_callback(IAsyncResult asyncResult)\r\n   
--- End of inner exception stack trace ---\r\n   
at Microsoft.Rtc.Internal.UCWeb.Utilities.AsyncHelper.EndAsyncCall[T](IAsyncResult asyncResult, String methodName, T ucwScopeInstance)\r\n   
at Microsoft.Rtc.Internal.UCWeb.UCWAuthenticatedEndpoint.EndSignIn(IAsyncResult asyncResult)\r\n   
at Microsoft.Exchange.Clients.Owa2.Server.Core.InstantMessageOCSProvider.<>c__DisplayClass33.<SignInCallback>b__32(RequestDetailsLogger logger)

The log files are located at

\Program Files\Microsoft\Exchange Server\V15\Logging\OWA\InstantMessaging

Solution

The Exchange Server OWA host name must be the common name (CN) of the SSL certificate used securing OWA communication.

Example for a non working IM configuration

  • OWA host name: owa.varunagroup.de
  • SSL certificate CN: mobile.varunagroup.de

Example for a working IM configuration

  • OWA host name: owa.varunagroup.de
  • SSL certificate CN: owa.varunagroup.de

Links

 

 

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