MS Teams with Exchange On-Premises

Frequent Contributor

MS Teams is unable to connect to Exchange calendar on Premises. The calendar icon is unavailable in Teams.
The mailbox is in Exchange on prem, there is a Hybrid configuration and Oauth is working.
The Calendar Sharing between On Premises and Cloud Users work.
The calendar is available in OWA.
All on prem users are affected.

Error message: "Your exchange calendar isn't set up. Talk to your admin to set up you calendar".

Exchange Server 2016 CU12

13 Replies

@AtanasM Did you check the external authentication methods on the AutoDiscover virtual directory?

It seems that Microsoft Teams cannot fetch AutoD version 2 data from your on-premises Exchange Servers. Are all Exchange virtual directories published to the Internet?  

@Thomas Stensitzki how to check the external authentication methods on the AutoDiscover virtual directory? Could you send me a guideline?

Hello@AtanasM ! 

Tried enabling Group-Writeback? 

Also make sure that OAuth is set up correct


Oliwer Sjöberg

Hello @oliwer_sjoberg 

I checked many times and Oauth is set up correctly.

Are you sure, that enabling Group-Writeback can solve the issue?

Thanks in advance.

best response confirmed by AtanasM (Frequent Contributor)

@AtanasM How is the on-premises Exchange Server published to the Internet? Is Windows Server 2016+ Application Proxy used, or another reverse proxy component?

In regard to the Exchange hybrid configuration, is AutoDiscover DNS used or does the Exchange hybrid configuration use a static TargetAutodiscoverEpr?

Hello@AtanasM ! 


If OAuth is set up correctly and your on-prem Exchange server(s) are published correctly towards the internet without special proxies or smarthosts. then Group writeback could be the issue here. It's worth a try :) 


I've previously enabled Group-Writeback and got this to work. 
Since you're in a hybrid, the local Exchange resources usualy needs a reference object in Active directory to route mail and work correctly. 

Read this Docs article for reference


Kind Regards
Oliwer sjöberg

Hello @oliwer_sjoberg 

The groups write back is already activated.
However, this has no influence on the access to the calendar from Teams and it is still not visible.
The Teams client tries to connect to the local Exchange but gets a response error 400.
This happens both in the WebClient and via the installed Teams client (verified with Fiddler).


ErrorCode: ErrorNonExistentMailbox, ErrorMessage: Mailbox doesn't exist; StatusCode 400


Please check the attachment.

Hello @AtanasM ! 

I see, the error message is strengthening the theory that @Thomas Stensitzki  Had regarding Autodiscover. 
The error message says that "Error while fetching apointments from EWS" And that the mailbox was not found. 

So there seems more likely that Autodiscover is the issue here. 
Is it possible for you to share your Autodiscover and EWS Virtual Directories configurations with us? 

Kind Regards

@Thomas Stensitzki The local exchange is addressed via a load balancer (KEMP Loadmaster). Inquiries to EWS etc. are passed on.

The HCW has filled out the TargetAutodiscoverEpr automatically.

from EXO to on premises and from on premises to EXO.

However, I have only tried it with registered target sharing EPR, but this did not lead to success.

@AtanasM I'd recommend the following steps for troubleshooting this further:



@oliwer_sjoberg I could share them in a private message. Is this possible for you?

Yes works for me :) @AtanasM 

@Thomas Stensitzki I added the local web service URLs as SPN in Azure AD and this solved the issue. For instructions, see the following link and section: “Add local web service URLs as SPNs in Azure AD”: