Today morning our Smlet Mail connector stopped processing mails from its mailbox. So no new work items were created, as well as no updates to existing work items were made. Looking at the event viewer logs showed, that no mails were processed from 04:20 onwards (CET). Setting the Smlets Exchange Connector logging to Level 4 showed the error cause immediately:
System.Management.Automation.MethodInvocationException: Exception calling "Bind" with "2" argument(s): "The request failed. The remote server returned an error: (403) Forbidden." ---> Microsoft.Exchange.WebServices.Data.ServiceRequestException: The request failed. The remote server returned an error: (403) Forbidden. ---> System.Net.WebException: The remote server returned an error: (403) Forbidden.....To rule out an Smlet Connector error, I tried enabling the stock connector. And it showed the same error message (just a bit shorter). I checked the Entra App which we created for the access some years ago and the log file showed successfull connections. It was also possible to access the mailbox via Outlook Online with the corresponding user. So it was clear, that the issue comes from EWS. We tried connecting to personal mailboxes and also connecting to the Connector Mailbox with EXO Admin Accounts and both worked. So it was clear, that it was no general EWS issue, it must've been connected to the user as well.
Then I came across this article: https://docs.revenuegrid.com/ri/fast/articles/Enabling-EWS-in-Exchange-Online-user-level/
We found out, that this EWS setting was disabled for our "connector user". Trying to set it to enabled failed because of a license error (From beginning the user for this mailbox had an Exchange Online Kiosk license and it has never been an issue). So we upgraded the license to Exchange Online Plan 1 and after that the connection worked immediately and all mails have been processed. What we know now it should've never worked with a Kiosk license, but until now it did. Seems like Microsoft has closed a gap quietly 😅
TL;DR: Connection via EWs from a third party application is only possible with Exchange online Plan 1 and above.
Maybe someone else came across the same error and finds a solution in my community post. 🙂
Adam Dzyacky As Microsoft stops connecting to EXO via EWS completely in October 2026 I wanted to ask if the Graph API SMLet Connector is in the works already.