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This blog is about mostly anything in IT. But the primary focuses are Microsoft technologies like Exchange Server, Microsoft 365, Microsoft Teams, and Cloud Security.

You might see the following error in the Windows Application Event Log:

  • Source: MSExchangeApplicationLogic
  • Event ID: 3018
  • Level: Error
The request failed. Mailbox:  

System.Net.WebException: The underlying connection was closed: An unexpected error occurred on a send. 
---> System.IO.IOException: Unable to read data from the transport connection: 
   An existing connection was forcibly closed by the remote host. 
---> System.Net.Sockets.SocketException: An existing connection was forcibly closed by the remote host
   at System.Net.Sockets.Socket.EndReceive(IAsyncResult asyncResult)
   at System.Net.Sockets.NetworkStream.EndRead(IAsyncResult asyncResult)
   --- End of inner exception stack trace ---
   at System.Net.TlsStream.EndWrite(IAsyncResult asyncResult)
   at System.Net.ConnectStream.WriteHeadersCallback(IAsyncResult ar)
   --- End of inner exception stack trace ---
   at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
   at Microsoft.Exchange.Data.ApplicationLogic.Extension.BaseAsyncOmexCommand.<>c__DisplayClass18_0.<EndGetResponseCallback>b__0()


Screenshot - Event log MSExchangeApplicationLogic Event ID 3018

The request is successful when you try to connect to the URL provided in the error details using a browser on the Exchange server. 



You can verify that the issue by trying to access the URL using the PowerShell Invoke-WebRequest cmdlet. Open a new PowerShell session and try connecting to the URL.


Invoke-WebRequest -Uri $uri

You will receive the same error message as stated in the event log by MSExchangeApplicationLogic. A successful connection returns XML as content.

The reason for this error is related to the .NET Framework TLS configuration, not Exchange Server. The .NET Framework lacks configuration for the use of TLS 1.2.



The solution for this issue is to configure the .NET Framework to correctly use TLS 1.2. You can follow the description for TLS 1.2 enforcement for Azure AD Connect, or you can simply use this Gist

Due to the changes made to the SCHANNEL configuration you just restart the computer to bring the changes into effect.


Changing the TLS settings does not only affect outgoing connections but incoming connections as well.

Test the TLS changes in a test environment before adjusting your servers in the production environment. If you have not already enabled TLS 1.2 for your Exchange Servers, I recommend reading the 3-part series by the Exchange product group.




Enjoy Exchange Server! 




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IllustrationThe Problem

Mail flow from on-premises devices and applications to Exchange Online is a tricky topic. The documentation allows for different solutions.

Recently a client ran into a situation where an on-premises application was not able to deliver messages to a configured inbound connector in the Exchange Online tenant. The connector was configured for remote IP address selection.

Exchange Online responded to each connection attempt with the following error message:

  • 451 4.4.3 Temporary server error. Please try again later ATTR3.1

There weren't any changes on the on-premises configuration and the setup was in use for multiple months without any issues.


The Solution

It took some time to identify the solution, but in the end, the solution was easy.

Disabling and re-enabling solved the issue.  


Enjoy Exchange Online.


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Exchange Server 2019 LogoThe Problem

You might face a situation during an Exchange Server migration where your Exchange Server 2019 mailbox users are not able to open their public folder favorites when using Outlook on the Web (OWA).

When your users try to access a public folder, they receive an error message.

Screenshot Public No Folders available


This error occurs when the public folder mailboxes are still hosted on a previous version of Exchange Server. This includes Exchange Server 2016 and 2013.

The online documentation explains, why this is happening:

  • Access public folders located on servers running previous versions of Exchange


The Solution

The solution to this problem is easy. Move the public folder mailboxes to Exchange Server 2019 before you migrate any user mailboxes. 

This approach ensures that mailboxes hosted on Exchange Server 2019 and previous versions of Exchange Server are able to access public folders using Outlook on the Web.




Enjoy Exchange Server.



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Microsoft 365 Collaboration BootCamp 2021The Microsoft 365 Collaboration BootCamp takes place on 21th & 21st  August 2021.

The event addresses collaboration and best practices for using Microsoft Teams, SharePoint, Lists, Groups, and Microsoft Security & Governance.

I am honored to speak about one of my favorite Topics: Microsoft Teams and On-Premises Mailboxes - Troubleshooting 101

Join my session on Saturday 21st August at 12:00 pm (GMT/UTC)







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The Exchange Product Group announced Exchange Server vNEXT for fall 2021. angekündigt. We are all very excited to see what the new version has to offer.

But what is the current situation in on-premises Exchange organizations? I have put a short questionnaire online for gathering information from you. 

The questionnaire deals with the currently used product versions of Exchange Server, the size of your Exchange organization in terms of the number of servers and mailboxes, and the use of planning for a hybrid configuration with Exchange Online.

Screenshot Exchange Server Questionnaire

Take the questionnaire following this link:

Thank you for your participation.


Viel Spaß mit Exchange Server.


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